Pass CFLAGS from our configure script to our defaults

This change adds CFLAGS and other FLAGS on to their configure scripts
from ours.

We also remove the make arguments, as I was originally going to pass
flags to make, but autoconf will add them. So this moves 'make
arguments' to 'configure flags'.

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
Ignore includes in all.mk if inc/defs.mk is missing

This allows clean and distclean to continue to run, even if they don't
do anything.

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
Add clean target, and add ACG_PREFIX to distclean

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>