reset CMAKE_REQUIRED_LIBRARIES before checking system headers

reset CMAKE_REQUIRED_LIBRARIES before checking system headers

This fixes this CMake warning with CMake 3.12 and newer:

CMake Warning (dev) at /usr/share/cmake/Modules/CheckIncludeFiles.cmake:110 (message):
  Policy CMP0075 is not set: Include file check macros honor
  CMAKE_REQUIRED_LIBRARIES.  Run "cmake --help-policy CMP0075" for policy
  details.  Use the cmake_policy command to set the policy and suppress this
  warning.

  CMAKE_REQUIRED_LIBRARIES is set to:

    /usr/lib/liblzma.so

  For compatibility with CMake 3.11 and below this check is ignoring it.
Call Stack (most recent call first):
  CMakeLists.txt:602 (CHECK_INCLUDE_FILES)
  CMakeLists.txt:609 (LA_CHECK_INCLUDE_FILE)