[PATCH] build: Honor user build flags

[PATCH] build: Honor user build flags

CPPFLAGS, CFLAGS and LDFLAGS are user flags, which we need to preserve,
regardless of the user passing them over the environment or the
command-line. Any required flag that the build system needs, has to be
set in some other flag (which we will namespace with «MK_») so that the
build will use even if the user flags are passed, even though the user
should be able to override by appending after them. We pass CPPFLAGS to
any compilation command that missed them.

Signed-off-by: Guillem Jover <guillem@hadrons.org>
Gbp-Pq: Name 0011-build-Honour-user-build-flags.patch