GNUmakefile: use make's $(CC) and allow a third-party to use whatever

When using distcc to help cross-compiling for platforms other than the
host's native arch, explicitly calling 'gcc' may lead to unfortunate
results, such as objects ending up being remotely compiled for x86_64
and then the local armv6 linker having no idea how to handle them.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>