autotools: Don't use "user variables" to set build flags

autotools: Don't use "user variables" to set build flags

GNU coding standards recommend against it and automake is designed
around it.

This allows overriding the global build flags using AM_CFLAGS, etc.,
or per object flags, something that is difficult or impossible currently
because of automake precedence rules.

Change-Id: I3f1ea12e560af5a46b2f04b342b1882bbf123f12
Reviewed-on: https://code.wireshark.org/review/13455
Petri-Dish: João Valverde <j@v6e.pt>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: João Valverde <j@v6e.pt>