each GCC version. Using the variable causes impossible version constraints
when a specific GCC is depended upon but the user is using something newer,
as _GCC_REQD will be set to the higher value.
Changes, New Features, and Fixes in GCC 4.9 can be found here:
https://gcc.gnu.org/gcc-4.9/changes.html
This is based heavily on lang/gcc47 but updated for GCC 4.9.1. Tested on
SmartOS (32-bit and 64-bit) and NetBSD 6.1.4/amd64.