title: CMAKE missing sysroot when cross compiling
body: <p>I am having some troubles setting up cross compiling with CMAKE. The toolchain I am using is created in yocto which works perfectly outside of cmake.</p>
  
  <p>I have followed a tutorial to setup the following toolchain file:</p>
  
  <pre><code>SET(CMAKE_SYSTEM_NAME Linux)
  SET(CMAKE_SYSTEM_VERSION 1)
  SET(CMAKE_SYSTEM_PROCESSOR arm)
  
  # specify the cross compiler
  SET(tools /opt/poky/1.7.1/sysroots/x86_64-pokysdk-linux/usr)
  SET(CMAKE_C_COMPILER ${tools}/bin/arm-poky-linux-gnueabi/arm-poky-linux-gnueabi-gcc)
  SET(CMAKE_CXX_COMPILER ${tools}/bin/arm-poky-linux-gnueabi/arm-poky-linux-gnueabi-gcc)
  
  # set sysroot
  SET(CMAKE_SYSROOT /home/sifu/test-yocto/qemuarmdfs)
  #SET(CMAKE_FIND_ROOT_PATH /home/sifu/test-yocto/qemuarm)
  
  # search for programs in the build host directories
  SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
  # for libraries and headers in the target directories
  SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
  SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
  SET(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
  </code></pre>
  
  <p>And get the following error when running cmake</p>
  
  <pre><code>The C compiler
  "/opt/poky/1.7.1/sysroots/x86_64-pokysdk-linux/usr/bin/arm-poky-linux-gnueabi/arm-poky-linux-gnueabi-gcc"
  is not able to compile a simple test program.
  
  It fails with the following output:
  
  Change Dir: /home/sifu/Projects/mv/doublepump-single-pump-sw.ss016m21_swapp/cc/CMakeFiles/CMakeTmp
  
  Run Build Command:/usr/bin/make "cmTryCompileExec4012536451/fast"
  
  /usr/bin/make -f CMakeFiles/cmTryCompileExec4012536451.dir/build.make
  CMakeFiles/cmTryCompileExec4012536451.dir/build
  
  make[1]: Entering directory
  `/home/sifu/Projects/mv/doublepump-single-pump-sw.ss016m21_swapp/cc/CMakeFiles/CMakeTmp'
  
  /usr/bin/cmake -E cmake_progress_report
  /home/sifu/Projects/mv/doublepump-single-pump-sw.ss016m21_swapp/cc/CMakeFiles/CMakeTmp/CMakeFiles
  1
  
  Building C object
  CMakeFiles/cmTryCompileExec4012536451.dir/testCCompiler.c.o
  
  
  /opt/poky/1.7.1/sysroots/x86_64-pokysdk-linux/usr/bin/arm-poky-linux-gnueabi/arm-poky-linux-gnueabi-gcc
  -O2 -pipe -g -feliminate-unused-debug-types -o
  CMakeFiles/cmTryCompileExec4012536451.dir/testCCompiler.c.o -c
  /home/sifu/Projects/mv/doublepump-single-pump-sw.ss016m21_swapp/cc/CMakeFiles/CMakeTmp/testCCompiler.c
  
  Linking C executable cmTryCompileExec4012536451
  
  /usr/bin/cmake -E cmake_link_script
  CMakeFiles/cmTryCompileExec4012536451.dir/link.txt --verbose=1
  
  /opt/poky/1.7.1/sysroots/x86_64-pokysdk-linux/usr/bin/arm-poky-linux-gnueabi/arm-poky-linux-gnueabi-gcc
  -O2 -pipe -g -feliminate-unused-debug-types -Wl,-O1 -Wl,--hash-style=gnu
  -Wl,--as-needed CMakeFiles/cmTryCompileExec4012536451.dir/testCCompiler.c.o
  -o cmTryCompileExec4012536451 -rdynamic
  
  ... (A lot of ld errors similar to the one below)
  
  /opt/poky/1.7.1/sysroots/x86_64-pokysdk-linux/usr/libexec/arm-poky-linux-gnueabi/gcc/arm-poky-linux-gnueabi/4.9.1/ld:
  cannot find crtn.o: No such file or directory
  
  collect2: error: ld returned 1 exit status
  
  make[1]: Leaving directory
  `/home/sifu/Projects/mv/doublepump-single-pump-sw.ss016m21_swapp/cc/CMakeFiles/CMakeTmp'
  </code></pre>
  
  <p>If I manually run the gcc command described in the log above with <strong>--sysroot=/home/sifu/test-yocto/qemuarmdfs</strong> it works for me. Why isn't cmake using the sysroot flag when the path to sysroot is added in the toolchain file.</p>