title: CMake Error: MYSQL_INCLUDE_DIR not found
body: <p>I am trying to establish connection with <code>MySQL</code> in C++. For that i am using <a href="https://github.com/rbock/sqlpp11" rel="nofollow">sqlpp11</a> and it's <a href="https://github.com/rbock/sqlpp11-connector-mysql" rel="nofollow">connector</a>.</p>
  
  <ol>
  <li><code>sqlpp11</code> successfully build.</li>
  <li><p><code>sqllpp11-connector-mysql</code> is not building properly because of some cmake error. </p>
  
  <p>CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
      Please set them or make sure they are set and tested correctly in the CMake files:
      MYSQL_INCLUDE_DIR (ADVANCED)</p></li>
  </ol>
  
  <p>This is how my my <code>CmakeList.txt</code> looks</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.2)
  include(CheckCXXSymbolExists)
  project (sqlpp11-connector-mysql)
  enable_testing()
  set(CMAKE_CXX_STANDARD 11)
  
  check_cxx_symbol_exists(_LIBCPP_VERSION iostream HAS_LIBCPP_VERSION)
  if (HAS_LIBCPP_VERSION)
      message("libc++ does not have thread_local, thus we need to wrap that via boost thread")
      find_package(Boost COMPONENTS thread)
  endif()
  
  
  set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
  find_package(MySql REQUIRED)
  
  message(STATUS "Using ${CMAKE_CXX_COMPILER} (compiler id: ${CMAKE_CXX_COMPILER_ID})")
  
  set(DATE_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../date" CACHE FILEPATH "Path to Howard Hinnant's date library")
  
  if(NOT EXISTS ${DATE_INCLUDE_DIR}/date.h)
      message(SEND_ERROR "Can't find file date.h")
      message("Can't find date.h in ${DATE_INCLUDE_DIR} ")
      message("Please either")
      message("  - git clone https://github.com/howardhinnant/date ${DATE_INCLUDE_DIR}")
      message("  - download and unzip a current version from https://github.com/howardhinnant/date to ${DATE_INCLUDE_DIR}")
      message("  - set DATE_INCLUDE_DIR to point to the dir containing date.h from the date library")
      message("")
  else()
      message("including date from ${DATE_INCLUDE_DIR}")
  endif()
  
  set(SQLPP11_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../sqlpp11/include" CACHE FILEPATH "Path to sqlpp11 includes")
  
  if(NOT EXISTS ${SQLPP11_INCLUDE_DIR}/sqlpp11/sqlpp11.h)
      message(SEND_ERROR "Can't find file sqlpp11/sqlpp11.h")
      message("Can't find sqlpp11/sqlpp11.h in ${SQLPP11_INCLUDE_DIR} ")
      message("Please either")
      message("  - git clone https://github.com/rbock/sqlpp11 ${SQLPP11_INCLUDE_DIR}")
      message("  - download and unzip a current version from https://github.com/rbock/sqlpp11 to ${SQLPP11_INCLUDE_DIR}")
      message("  - set DATE_INCLUDE_DIR to point to the dir containing sqlpp11/sqlpp11.h")
      message("")
  else()
      message("including sqlpp11 from ${SQLPP11_INCLUDE_DIR}")
  endif()
  
  set(MYSQL_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../include/sqlpp11/mysql" CACHE FILEPATH "PATH TO sqllpp11/mysql")
  
  include_directories("${SQLPP11_INCLUDE_DIR}")
  include_directories("${DATE_INCLUDE_DIR}")
  include_directories("${MYSQL_INCLUDE_DIRS}")
  set(include_dir "${PROJECT_SOURCE_DIR}/include")
  file(GLOB_RECURSE sqlpp_headers ${include_dir}/*.h ${SQLPP11_INCLUDE_DIR}/*.h)
  include_directories(${include_dir})
  
  add_subdirectory(src)
  add_subdirectory(tests)
  
  install(DIRECTORY "${PROJECT_SOURCE_DIR}/include/sqlpp11" DESTINATION include)
  </code></pre>