xrdb: Use mcpp as the preprocessor

Close #9501, fixes #9480.

By default, xrdb uses GCC as the preprocessor at runtime for X resource files.
However, gcc is a large dependency, so replace it with mcpp, a much smaller
preprocessor (currently under a megabyte on i686).

Arch Linux already does this as well, so this should be relatively safe:
https://projects.archlinux.org/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/xorg-xrdb