Makefile: Define CFLAGS_FORCE_C_SOURCE macro

This macro holds the C compiler flag(s) to force input files to be
recognized as C sources regardless of extension.  It is defined to allow
overriding on the make command line.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>