[PATCH] build: Don't add _FORTIFY_SOURCE if already set by user/toolchain

[Sam James]

Newer toolchains (GCC 12+ or Clang 9+, glibc-2.34) allow _FORTIFY_SOURCE=3.

The current macro used in configure.ac will forcefully downgrade to F_S=2
and emit a warning if the user set something else:
```
x86_64-pc-linux-gnu-gcc -DEXEEXT=\"\" -I. -I../lib  -DIN_M4_GNULIB_TESTS=1 -I. 
-I. -I.. -I./.. -I../lib -I./../lib   -O2 -pipe -march=native 
-fdiagnostics-color=always -frecord-gcc-switches -Wreturn-type      -ggdb3 
-Werror=implicit-function-declaration -Werror=implicit-int -c -o 
glthread/thread.o glthread/thread.c
In file included from glthread/thread.c:20:
../lib/config.h:202: warning: "_FORTIFY_SOURCE" redefined
  202 |    # define _FORTIFY_SOURCE 2
      |
<built-in>: note: this is the location of the previous definition
```

See: 390d259efe8e1c7e4b6babb4738fef7427416857
Signed-off-by: Sam James <sam@gentoo.org>
---
 configure.ac | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index efa57d2e..574954bc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -133,7 +133,9 @@ if test "$gl_gcc_warnings" = yes; then
   [/* Enable compile-time and run-time bounds-checking, and some warnings,
       without upsetting newer glibc. */
    #if defined __OPTIMIZE__ && __OPTIMIZE__
-   # define _FORTIFY_SOURCE 2
+   # ifndef _FORTIFY_SOURCE
+   #  define _FORTIFY_SOURCE 2
+   # endif
    #endif
   ])
 fi
-- 
2.39.0