Fix Yosemite builds that use gcc-4.2 through `require_gcc`

The definitions that use `require_gcc` are not compatible with Apple's
clang-powered `gcc` and need gcc-4.2 from Homebrew. However, builds
using gcc-4.2 fail on Yosemite with a warning:

    couldn't understand kern.osversion `14.0.0'

Although the warning is non-fatal, the build goes to shit from there. It
seems that setting the magical value `MACOSX_DEPLOYMENT_TARGET=10.9`
makes the build work and doesn't seem to have negative consequences.