Problem

NIX_CFLAGS_COMPILE is a low-level hack that sneaks compiler arguments past packages' build systems.

It has various drawbacks:

• the build system cannot see the flag, thus intended upstream logic in the build system is potentially not activated
• generated files, such as pkg-config .pc files, may lack the necessary entries, making problems for downstream packages
• the flag does not show up in the build logs, making it unclear what's happening

Because many packages use NIX_CFLAGS_COMPILE, more packages get it added when people copy this style.

Solution

It is better to pass CFLAGS, CXXFLAGS and so on directly to the build system via the method intended to pass compiler flags, for example (feel free to edit this to add more build systems):

• Autoconf: By setting environment variables like CFLAGS before the ./configure invocation (e.g. by writing CFLAGS = "..."; into mkDerivation)
• CMake: By setting environment variables like CFLAGS before the cmake invocation (see also cmake setup hook: Allow passing environment variables to cmake invocation #79302)
• Meson: -Dc_args (see also cinnamon.nemo: init at 4.4.1 #77979 (comment) and Report undeclared gio-unix-2.0 upstream #36468 (comment))

Delivery

I think we should run a nixpkgs-wide sprint (like #79064) to do this switch for as many packages as possible.

We should also update the documentation to recommend better alternatives to this hack (like above).