Fall back to C99 when C11 isn't available
Variables are case-sensitive.

You only see this on platforms like NetBSD where /usr/pkg is not even
close to being included in the compiler's default paths.
add -fms-extensions for compatibility with old GCC
Merge pull request #2 from awilfox/netbsd-support

NetBSD (and OS X 10.4/10.5) support