build: allow kmscon to be cross-compiled

The genshader and genunifont utilities are run during the build process
to generate source files. In order for that to work when cross-compiling
the files need to be built using the native compiler instead of the
cross-compiler.

Add the AX_PROG_CC_FOR_BUILD m4 macro which defines various *_FOR_BUILD
variables that are the native equivalents of CC, CFLAGS, LDFLAGS, etc.
Override CC, CFLAGS and LDFLAGS for genshader and genunifont and their
object files so that they will be built natively and can be executed
during the build.

Signed-off-by: Thierry Reding <treding@nvidia.com>
(added TODO marker)
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>