fix CMake CMP0075 warning

Signed-off-by: Steffen Pankratz <[email protected]>

Original warning:
  CMake Warning (dev) at /usr/share/cmake-3.19/Modules/CheckIncludeFile.cmake:80 (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:

    pthread;rt

  For compatibility with CMake 3.11 and below this check is ignoring it.
Call Stack (most recent call first):
  CMakeLists.txt:56 (check_include_file)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMakeLists.txt

@@ -1,5 +1,11 @@
 cmake_minimum_required(VERSION 2.8.12 FATAL_ERROR)
 
+# CMP0075 Include file check macros honor CMAKE_REQUIRED_LIBRARIES
+# For more information see: https://cmake.org/cmake/help/latest/policy/CMP0075.html
+if(POLICY CMP0075)
+    cmake_policy(SET CMP0075 NEW)
+endif()
+
 file(STRINGS "VERSION" VERSION)
 
 project(ebusd)