title: Use -isystem instead of -I with CMake
body: <p>Is there any way in CMake to force a path specified via include_directories (or perhaps through a different function) to use the -isystem flag instead of the -I flag when building with gcc?</p>
  
  <p>See <a href="http://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html#Directory-Options" rel="noreferrer">http://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html#Directory-Options</a> for details on -I and -isystem.</p>