Allow ICU's version.mk to be used by GNU make (= instead of *=).
Add support for building generated C files.
Reference ICU libraries using "gb_Library_use_externals" across the tree. Also ICU is an external library, remove it from RepositoryFixes.mk which is only for internal ones, and keep it in RepositoryExternal.mk exclusively. Since we get to specify the full linker parameter over there, remove the half-baked hacks for it from RepositoryFixes.mk.