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)
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Merged revisions 421227 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 421228 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 421229 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 421230 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@421231 f38db490-d61c-443f-a65b-d21fe96a405b