Build fails if -D_FORTIFY_SOURCES is in env CFLAGS

[real-or-random]

Describe the bug
setup.py prepends the env CFLAGS with -D_FORTIFY_SOURCES=2. This leads to a build failure when the env CFLAGS contain a different definition. For example, Arch Linux recently switched to -D_FORTIFY_SOURCES=3 (see https://rfc.archlinux.page/0017-increase-fortification-level/). When I try to build the Arch AUR package kitty-git, I get this (after modifying setup.py not to suppress stderr):

<command-line>: error: "_FORTIFY_SOURCE" redefined
<command-line>: note: this is the location of the previous definition
Failed to determine target architecture compiling test program failed with exit code: 1  

See https://aur.archlinux.org/packages/kitty-git#comment-961665 for more reports of this.

Analysis/Suggestion
This is a bit unfortunate. Usually, later CFLAGS override earlier CFLAGS (to give the user the last word), but not in the case of -D. (Strictly speaking, -D_FORTIFY_SOURCES=2 belongs to CPPFLAGS, but it probably won't matter, and even Arch Linux's defaults get this wrong... edit: Apparently this is there for good reasons: https://rfc.archlinux.page/0003-buildflags/)

I suggest

• adding -D_FORTIFY_SOURCES=2 only if CFLAGS and CPPFLAGS don't contain the substring -D_FORTIFY_SOURCES=
• switching to -D_FORTIFY_SOURCES=3 (this alone would make the problem go away on Arch Linux, but it's not a proper solution...)

[kovidgoyal]

I am on Arch too, it doesnt set CFLAGS as an env var. This must be
something specific to the kitty-git aur build script. That said, easy
enough to fix.

[real-or-random]

I am on Arch too, it doesnt set CFLAGS as an env var. This must be
something specific to the kitty-git aur build script.

Okay, strictly speaking, Arch doesn't set it, but makepkg does: https://gitlab.archlinux.org/archlinux/packaging/packages/pacman/-/blob/main/makepkg.conf?ref_type=heads

[CanadianBaconBoi]

The kitty-git package builds perfectly when manually running makepkg -si. The yay AUR helper is where the issue seems to lie.

[real-or-random]

The kitty-git package builds perfectly when manually running makepkg -si. The yay AUR helper is where the issue seems to lie.

Okay, I don't know. I haven't tried this. Anyway, it's fixed now. :)

[HanabishiRecca]

@kovidgoyal doesn't 20375ee break OVERRIDE_CFLAGS?

[kovidgoyal]

It prevents it from overriding fortify source yes, but there are many
other things it doesnt override anyway.