build: avoid annoying _FORTIFY_SOURCE warnings (fixes #17341)

fixed patch with the duplicate "#if defined ..." removed

Janne
---8<---
Some toolchains predefine _FORTIFY_SOURCE resulting in countless
_FORTIFY_SOURCE is redefined warnings. Using _FORTIFY_SOURCE without
compiler optimizations also generates warnings.
_FORTIFY_SOURCE is a reserved identifier in C99 ("All identifiers that
begin with an underscore and either an uppercase letter or another
underscore are always reserved for any use.") so the toolchain is
perfectly free to predefine it.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>