Log Message:
glibmm: Drop GCC_REQD from buildlink3.mk

As a hack for 2017Q4, I added GCC_REQD+=4.9 to glibmm's buildlink3.mk,
to make some fraction of glibmm-depending packages build on the
branch.  This commit reverts the bl3 addition (but leaves
GCC_REQD+=4.9 for glibmm itself).

The result will be that someone building pkgsrc with a compiler older
than gcc 4.9 will be able to build glibmm (via forcing 4.9), and
depending packages will fail.  This is much like the situation before
the previous commit, except that glibmm will build.