build: Turn FORTIFY_SOURCE off if DONT_OPTIMIZE is set.

build:  Turn FORTIFY_SOURCE off if DONT_OPTIMIZE is set.

AST_FORTIFY_SOURCE is automatically set in ./Makefile even if DONT_OPTIMIZE
is set in menuselect.  This causes gcc to complain that _FORTIFY_SOURCE
requires optimization and the build will fail.  You can specify
"make AST_FORTIFY_SOURCE=''" but I always forget.

This patch moves the set of AST_FORTIFY_SOURCE to Makefile.rules and only
sets it if DONT_OPTIMIZE is "no".  The move is necessary because the
top-level Makefile doesn't include menuselect.makeopts.

This doesn't solve the entire problem however because res_config_mysql
seems to force _FORTIFY_SOURCE so res_config_mysql has to be disabled
for now if DONT_OPTIMIZE is set.

Tested by: George Joseph
Review: https://reviewboard.asterisk.org/r/3664/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@417016 65c4cc65-6c06-0410-ace0-fbb531ad65f3