[arch-general] CMake silently disregards the C preprocessor flags environment variable

[Neven Sajko]

http://www.cmake.org/Bug/view.php?id=12928
Did you know about this? And this is an error in design, it has been
like this since the beginning of CMake. It seems like a pretty shameful
error for a build system.

Although patching for every package using Cmake is probably out of the
question, I really feel like this is an issue something should be done
about.