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, 1 insertions, 1 deletions

diff --git a/config.mk.in b/config.mk.in
index 253b86435..a7f66fbb5 100644
--- a/config.mk.in
+++ b/config.mk.in
@@ -91,7 +91,7 @@ COMMON_CFLAGS = \
         -ggdb -O2 -DVERSION='"$(VERSION)"' \
         -Wall -Wextra $(HAVE_FATAL_WARNINGS) \
         -Wformat -Wformat-security \
-        -fstack-protector-all -D_FORTIFY_SOURCE=2 \
+        -fstack-protector-all \
         -DPREFIX='"$(prefix)"' -DSYSCONFDIR='"$(sysconfdir)/firejail"' \
         -DLIBDIR='"$(libdir)"' -DBINDIR='"$(bindir)"' \
         -DVARDIR='"/var/lib/firejail"'