The Win32 build was not taking the new path of git-version.h into
account and was outputting these errors:
> ../build/windows/gimp.rc:2:10: fatal error: git-version.h: No such file or directory
> #include "git-version.h"
> ^~~~~~~~~~~~~~~
This step moves the %.rc.o pattern rule to a small include file in the
build/windows/ directory. Michael Natterer suggested this approach in
order to minimize the changes done to each executable's Makefile.am.
