Makefile: Respect "CC" environment variable.

There are cases where someone might want to use another compiler besides GCC.
There are also automated build systems (like Yocto) that depend on being able
to override CC.

Signed-off-by: Max Eliaser <max.eliaser@intel.com>
Merge pull request #50 from Max-E/master

Makefile: Respect "CC" environment variable.