build: fix "warning: "_FORTIFY_SOURCE" redefined" (#6283)

The warning is being produced on Arch since pacman 6.1, which changed
`-D_FORTIFY_SOURCE=2` to `-D_FORTIFY_SOURCE=3` in CFLAGS in
makepkg.conf:

    $ pacman -Q gcc pacman
    gcc 13.2.1-5
    pacman 6.1.0-3
    $ makepkg
    [...]
    make -C src/lib
    gcc [...] -D_FORTIFY_SOURCE=2 [...] -Wp,-D_FORTIFY_SOURCE=3 [...] -c ../../src/lib/common.c -o ../../src/lib/common.o
    <command-line>: warning: "_FORTIFY_SOURCE" redefined
    <command-line>: note: this is the location of the previous definition

To fix this, only add `-D_FORTIFY_SOURCE` to EXTRA_CFLAGS if it does not
cause any warnings with CFLAGS and CPPFLAGS during compilation.

The effect remains the same: The build system still defines the macro by
default (if there are no warnings) and the user/distribution can still
override it through CFLAGS/CPPFLAGS.

Fixes #6282.

Reported-by: @glitsj16

1 files changed, 4 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 2762c2ef0..73bd334f8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -30,6 +30,10 @@ AX_CHECK_COMPILE_FLAG([-MMD -MP], [
         DEPS_CFLAGS="$DEPS_CFLAGS -MMD -MP"
 ])
 
+AX_CHECK_COMPILE_FLAG([-D_FORTIFY_SOURCE=2], [
+        EXTRA_CFLAGS="$EXTRA_CFLAGS -D_FORTIFY_SOURCE=2"
+], [], [$CFLAGS $CPPFLAGS -Werror])
+
 HAVE_SPECTRE="no"
 AX_CHECK_COMPILE_FLAG([-mindirect-branch=thunk], [
         HAVE_SPECTRE="yes"