glibmm: Add kludge to use gcc 4.9 for depending packages

glibmm seems to need gcc 4.9 to build; therefore set GCC_REQD.

This means that depending packages using other versions (specifically
4.8 on NetBSD 7) will fail to link.  As a kludge to make more things
build for the upcoming branch, force GCC_REQD to 4.9 in glibmm's
buildlink3.mk.  Now, at least some packages that depend on glibmm
build.  While icky, it's my judgement that having more packages
building is better.

This will very likely be reverted either after the branch or as part
of the upcoming compiler selection logic changes.

As proposed on tech-pkg.

(gdt)