configure: Undefine FORTIFY_SOURCE prior to defining it for patched gcc

configure: Undefine FORTIFY_SOURCE prior to defining it for patched gcc

Some distributions of Linux patch gcc to define FORTIFY_SOURCE when gcc is
executed with optimization. This "help" unfortunately results in re-definition
warnings when FORTIFY_SOURCE is later defined in Asterisk's build system. This
patch undefines FORTIFY_SOURCE prior to defining it to prevent this warning.

Review: https://reviewboard.asterisk.org/r/3912/

ASTERISK-24032 #close
Reported by: Kilburn
Tested by: Kilburn, wdoekes
patches:
  1.8.diff uploaded by cloos (License 5956)
  10.diff uploaded by cloos (License 5956)
  11.diff uploaded by cloos (License 5956)
  12.diff uploaded by cloos (License 5956)
  13.diff uploaded by cloos (License 5956)
........

Merged revisions 421227 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 421228 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@421229 65c4cc65-6c06-0410-ace0-fbb531ad65f3