Fix check for gcc version

This version should work on Mac OS X and other *BSD systems.

I removed the !clang check, because recent command-line tools for Mac OS
X report a version of 4.2.1 (Yosemite). I really only want to know that
we have at least 4.2.0, and a separate variable to know if it is gcc or
really clang.

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>