title: Conan create package with dependencies
body: <p>I'm trying to build a conan package for netcdf-c. NetCDF requires zlib and hdf5 as dependencies, both of them are available through the conan center. </p>
  
  <p>I added them as requirements to my conanfile.py. When running <code>conan create . testing/build</code> to create the conan package from my conanfile.py CMake is not able to build netcdf-c. The output of <code>conan create</code> shows me that zlib and hdf5 are installed, but can't be found. When compiling netcdf-c with cmake I have to set configure it like this: <code>CPPFLAGS="-I${H5DIR}/include -I${ZDIR}/include" LDFLAGS="-L${H5DIR}/lib -L${ZDIR}/lib" --prefix=${NCDIR} --disable-dap</code>. How can I set these flags inside my <code>conanfile.py</code> ?</p>
  
  <p>conanfile.py:</p>
  
  <pre><code>from conans import ConanFile, CMake, tools
  
  
  class NetcdfcConan(ConanFile):
      name = "netcdf_c"
      version = "4.7.3"
      license = "BSD 3-Clause 'New' or 'Revised' License"
      author = "Arne Osterthun arne.osterthun@dlr.de"
  
      description = "The Unidata network Common Data Form (netCDF) is an interface for scientific data access and a freely-distributed software library that provides an implementation of the interface. The netCDF library also defines a machine-independent format for representing scientific data. Together, the interface, library, and format support the creation, access, and sharing of scientific data. The current netCDF software provides C interfaces for applications and data. Separate software distributions available from Unidata provide Java, Fortran, Python, and C++ interfaces. They have been tested on various common platforms."
      topics = ("raster data", "ndarray", "array")
      settings = "os", "compiler", "build_type", "arch"
      options = {"shared": [True, False]}
      default_options = {"shared": False}
      requires = "zlib/1.2.11", "hdf5/1.10.6"
      generators = "cmake"
  
      def source(self):
          self.run("git clone --branch v4.7.3 https://github.com/Unidata/netcdf-c.git")
  
      def build(self):
          cmake = CMake(self)
          cmake.configure(source_folder="netcdf-c")
          cmake.build()
  
      def package(self):
          pass
  
      def package_info(self):
          pass
  
  </code></pre>
  
  <p>output:</p>
  
  <pre><code>[HOOK - attribute_checker.py] pre_export(): WARN: Conanfile doesn't have 'url'. It is recommended to add it as attribute
  Exporting package recipe
  netcdf_c/4.7.3@testing/build: A new conanfile.py version was exported
  netcdf_c/4.7.3@testing/build: Folder: /home/domane/.conan/data/netcdf_c/4.7.3/testing/build/export
  netcdf_c/4.7.3@testing/build: Package recipe modified in export, forcing source folder removal
  netcdf_c/4.7.3@testing/build: Use the --keep-source, -k option to skip it
  netcdf_c/4.7.3@testing/build: Exported revision: d70e63d07a15cf7376055e606b3244c3
  Configuration:
  [settings]
  arch=x86_64
  arch_build=x86_64
  build_type=Release
  compiler=gcc
  compiler.libcxx=libstdc++11
  compiler.version=7
  os=Linux
  os_build=Linux
  [options]
  [build_requires]
  [env]
  
  netcdf_c/4.7.3@testing/build: Forced build from source
  netcdf_c/4.7.3@testing/build (test package): Installing package
  Requirements
      hdf5/1.10.6 from 'conan-center' - Cache
      netcdf_c/4.7.3@testing/build from local cache - Cache
      zlib/1.2.11 from 'conan-center' - Cache
  Packages
      hdf5/1.10.6:d1d93931f0a5aab8450858e1d6eae88ac6c39f6d - Cache
      netcdf_c/4.7.3@testing/build:ed9c760b5cbeeac4d6f9420203834fe68a6384de - Build
      zlib/1.2.11:d50a0d523d98c15bb147b18fa7d203887c38be8b - Cache
  
  Installing (downloading, building) binaries...
  zlib/1.2.11: Already installed!
  hdf5/1.10.6: Already installed!
  netcdf_c/4.7.3@testing/build: WARN: Build folder is dirty, removing it: /home/domane/.conan/data/netcdf_c/4.7.3/testing/build/build/ed9c760b5cbeeac4d6f9420203834fe68a6384de
  netcdf_c/4.7.3@testing/build: Configuring sources in /home/domane/.conan/data/netcdf_c/4.7.3/testing/build/source
  Cloning into 'netcdf-c'...
  Note: checking out 'b7cd387bee8c661141fabb490f4969587c008c55'.
  
  You are in 'detached HEAD' state. You can look around, make experimental
  changes and commit them, and you can discard any commits you make in this
  state without impacting any branches by performing another checkout.
  
  If you want to create a new branch to retain commits you create, you may
  do so (now or later) by using -b with the checkout command again. Example:
  
    git checkout -b &lt;new-branch-name&gt;
  
  Checking out files: 100% (2211/2211), done.
  netcdf_c/4.7.3@testing/build: Copying sources to build folder
  netcdf_c/4.7.3@testing/build: Building your package in /home/domane/.conan/data/netcdf_c/4.7.3/testing/build/build/ed9c760b5cbeeac4d6f9420203834fe68a6384de
  netcdf_c/4.7.3@testing/build: Generator cmake created conanbuildinfo.cmake
  netcdf_c/4.7.3@testing/build: Calling build()
  -- The C compiler identification is GNU 7.5.0
  -- Check for working C compiler: /usr/bin/cc
  -- Check for working C compiler: /usr/bin/cc -- works
  -- Detecting C compiler ABI info
  -- Detecting C compiler ABI info - done
  -- Detecting C compile features
  -- Detecting C compile features - done
  -- Check if the system is big endian
  -- Searching 16 bit integer
  -- Looking for sys/types.h
  -- Looking for sys/types.h - found
  -- Looking for stdint.h
  -- Looking for stdint.h - found
  -- Looking for stddef.h
  -- Looking for stddef.h - found
  -- Check size of unsigned short
  -- Check size of unsigned short - done
  -- Using unsigned short
  -- Check if the system is big endian - little endian
  -- Performing Test LIBTOOL_HAS_NO_UNDEFINED
  -- Performing Test LIBTOOL_HAS_NO_UNDEFINED - Success
  -- Performing Test CC_HAS_WCONVERSION
  -- Performing Test CC_HAS_WCONVERSION - Success
  -- Performing Test CC_HAS_SHORTEN_64_32
  -- Performing Test CC_HAS_SHORTEN_64_32 - Failed
  -- Unable to determine HDF5 C flags from HDF5 wrapper.
  -- Unable to determine HDF5 C version from HDF5 wrapper.
  CMake Error at /usr/share/cmake-3.10/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
    Could NOT find HDF5 (missing: HDF5_LIBRARIES HDF5_INCLUDE_DIRS
    HDF5_HL_LIBRARIES C HL) (found version "")
  Call Stack (most recent call first):
    /usr/share/cmake-3.10/Modules/FindPackageHandleStandardArgs.cmake:378 (_FPHSA_FAILURE_MESSAGE)
    /usr/share/cmake-3.10/Modules/FindHDF5.cmake:905 (find_package_handle_standard_args)
    CMakeLists.txt:623 (FIND_PACKAGE)
  
  
  -- Configuring incomplete, errors occurred!
  See also "/home/domane/.conan/data/netcdf_c/4.7.3/testing/build/build/ed9c760b5cbeeac4d6f9420203834fe68a6384de/netcdf-c/build/CMakeFiles/CMakeOutput.log".
  See also "/home/domane/.conan/data/netcdf_c/4.7.3/testing/build/build/ed9c760b5cbeeac4d6f9420203834fe68a6384de/netcdf-c/build/CMakeFiles/CMakeError.log".
  netcdf_c/4.7.3@testing/build: 
  netcdf_c/4.7.3@testing/build: ERROR: Package 'ed9c760b5cbeeac4d6f9420203834fe68a6384de' build failed
  netcdf_c/4.7.3@testing/build: WARN: Build folder /home/domane/.conan/data/netcdf_c/4.7.3/testing/build/build/ed9c760b5cbeeac4d6f9420203834fe68a6384de
  ERROR: netcdf_c/4.7.3@testing/build: Error in build() method, line 22
          self.run("cd netcdf-c &amp;&amp; mkdir build &amp;&amp; cd build &amp;&amp; cmake .. &amp;&amp; cmake --build . -- -j8", run_environment=True)
          ConanException: Error 1 while executing cd netcdf-c &amp;&amp; mkdir build &amp;&amp; cd build &amp;&amp; cmake .. &amp;&amp; cmake --build . -- -j8
  </code></pre>