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.
Silence warnings when testing gccs

This silences the warning that gcc-4.2 produces on Yosemite:

    couldn't understand kern.osversion `14.0.0'
Use CC=clang by default on Yosemite to avoid gcc-4.2

Newer MRIs will pick up gcc-4.2 from PATH and use that instead of
`/usr/bin/gcc`. While this worked up till now, it will not work in
Yosemite anymore since Homebrew's apple-gcc42 is generally not
compatible with 10.10.

So when CC has not explicitly been set, set it to `clang` to avoid
searching the PATH for any other gcc versions. This fixes MRI builds on
systems where apple-gcc42 is installed.
Unset CC to isolate from CI build environment
Merge branch 'yosemite'

Fixes #651, fixes #648