[Cmake] -I/usr/include needed before -I/usr/X11R6/include

[Brad King]

> I need to have -I/usr/include before -I/usr/X11R6/include on the c++
> compile command so I get GL/*.h from /usr/include/GL, and not
> /usr/X11R6/include/GL.  Is there a way to specify this in cmake?  I
> tried the following
> 
>   INCLUDE_DIRECTORIES( /usr/include ) # ignored

Compiler writers tell us to NEVER use "-I /usr/include" because it screws
with the compiler's library header files.  Some compilers replace system
headers by placing them in an internal include directory that comes before
/usr/include.  If the user specified -I /usr/include, it breaks this
design.  CMake always ignores /usr/include as an include directory
specifically for this reason.

-Brad