Don't let Makefile.defs complain about old compiler when using gcc 7.

This also gives gcc 7 the same default values as the other recent gcc versions. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/1282

-- Commit Summary --

* Support gcc 7+.

-- File Changes --

M src/Makefile.defs (70)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/1282.patch https://github.com/kamailio/kamailio/pull/1282.diff