title: cmake cross compile can not find header file
body: <p>I want to cross compile <strong>amazon Alexa SDK</strong> in openwrt platform, but it can not find <code>curl/curl.h</code> when excute <code>make</code>, Here is my operate step:</p>
  
  <ol>
  <li><p>create a cmake toolchain file like this:</p>
  
  <pre><code>SET(CMAKE_SYSTEM_NAME Linux)
  SET(CMAKE_SYSTEM_VERSION 1)
  SET(CMAKE_C_COMPILER  arm-openwrt-linux-muslgnueabi-gcc)
  SET(CMAKE_CXX_COMPILER  arm-openwrt-linux-muslgnueabi-g++)
  
  SET(CMAKE_FIND_ROOT_PATH  /home/sundq/workspace/Develop/out/astar-parrot/staging_dir/target/rootfs)
  
  SET(CMAKE_SYSROOT  /home/sundq/workspace/Develop/out/astar-parrot/staging_dir/target/rootfs)
  
  SET(CMAKE_LIBRARY_PATH  /home/sundq/workspace/Develop/out/astar-parrot/staging_dir/target/rootfs/usr/lib)
  
  SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
  SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
  SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
  
  SET(CMAKE_C_FLAGS "-I/home/sundq/workspace/Develop/out/astar-parrot/staging_dir/target/rootfs/usr/include")
  
  SET(CMAKE_CXX_FLAGS "-I/home/sundq/workspace/Develop/out/astar-parrot/staging_dir/target/rootfs/usr/include")
  </code></pre></li>
  <li><p>excute <code>cmake</code> like this:</p>
  
  <pre><code>cmake ../avs-device-sdk-master-v1.0.2/ -DCMAKE_BUILD_TYPE=MINSIZEREL -DCMAKE_TOOLCHAIN_FILE=../toolchain.cmake
  </code></pre></li>
  <li><p>excute <code>make</code>, the output is:</p>
  
  <pre><code>/AVSCommon/AVS/src/AlexaClientSDKInit.cpp:18:23: fatal error: curl/curl.h: No such file or directory
  </code></pre></li>
  </ol>
  
  <p>I'm sure the curl/curl.h file exist in <code>/home/sundq/workspace/Develop/out/astar-parrot/staging_dir/target/rootfs/usr/include</code> directory.</p>