optflags: set _FORTIFY_SOURCE for preproc

optflags: set _FORTIFY_SOURCE for preproc

Without it will fail if optflags are not overridden
and raise an error like :

  configure: CFLAGS error: CFLAGS may only be used to specify C compiler flags, not macro definitions. Use CPPFLAGS for: -D_FORTIFY_SOURCE=2
  configure: error: Can not continue. Fix errors mentioned immediately above this line.
  error: Bad exit status from /var/tmp/rpm-tmp.AoOYbS (%build)

More details at :

https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Preprocessor-Options.html#index-Wp-986

Change-Id: I49c8545a0526a1922905ef55de3a59edf9036068
Signed-off-by: Philippe Coval <philippe.coval@open.eurogiciel.org>