Fix PR gdb/23835: Don't redefine _FORTIFY_SOURCE if it's already defined

Fix PR gdb/23835: Don't redefine _FORTIFY_SOURCE if it's already defined

Gentoo has a local GCC patch which always defines _FORTIFY_SOURCE=2.
This causes a build problem when building GDB there, because
"common/common-defs.h" also defines _FORTIFY_SOURCE=2:

    CXX    gdb.o
  In file included from ../../gdb/defs.h:28:0,
		   from ../../gdb/gdb.c:19:
  ../../gdb/common/common-defs.h:71:0: error: "_FORTIFY_SOURCE" redefined [-Werror]
   #define _FORTIFY_SOURCE 2

  <built-in>: note: this is the location of the previous definition
  cc1plus: all warnings being treated as errors
  make[2]: *** [Makefile:1619: gdb.o] Error 1

Even though it is questionable whether Gentoo's approach is the
correct one:

  https://jira.mongodb.org/browse/SERVER-29982
  https://bugs.gentoo.org/621036

it is still possible for GDB to be a bit more robust here and make
sure it just defines _FORTIFY_SOURCE if it hasn't been defined
already.  This patch does that.

Tested by rebuilding and making sure the macro was defined.

gdb/ChangeLog:
2018-10-31  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/23835
	* common/common-defs.h: Don't redefine _FORTIFY_SOURCE if it's
	already defined.