Add a variable to handle extra CFLAGS values

Add a variable to handle extra CFLAGS values

Some packages don't install its headers in the default directory
(e.g.: In Arch Linux libiberty and libn13 includes are installed)
in their own subdirectory under /usr/include) and the build fails
trying to find the headers.

This patch will allow passing extra CFLAGS values without discarding
the assignments made in the Makefile. The CFLAGS values in the Makefile
are ignored, if defined directly in the make command line.

Signed-off-by: Roger Zanoni <roger.zanoni@openbossa.org>