mk: stop on warning only in developer build

mk: stop on warning only in developer build

Add RTE_DEVEL_BUILD make-variable which can be used to do things
differently when doing development vs building a release,
autodetected from source root .git presence and overridable via
commandline. It is used it to enable -Werror compiler flag and may
be extended to other checks.

Failing build on warnings is a useful developer tool but its bad
for release tarballs which can and do get built with newer
compilers than what was used/available during development. Compilers
routinely add new warnings so code which built silently with cc X
might no longer do so with X+1. This doesn't make the existing code
any more buggier and failing the build in this case does not help
to improve the quality of an already released version either.

This change the default flags which can be tuned with EXTRA_CFLAGS.

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>