GCC14 compat: Narrower, more cross-GCC warning disablement

- moira only actually needs to disable one of the three warnings, so do
  that
- Older GCC doesn't know about return-mismatch
  (https://gcc.gnu.org/gcc-14/porting_to.html#return-mismatch), so we
  need to use the -Wno-return-mismatch form to compile with older GCC
  (see https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html), not
  -Wno-error=return-mismatch (which generates an unknown warning error)

Author: dehnert

pkgs/discuss.nix

@@ -17,7 +17,7 @@ in stdenv.mkDerivation {
   };
 
   env = {
-    NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration -Wno-error=implicit-int -Wno-error=return-mismatch";
+    NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration -Wno-error=implicit-int -Wno-return-mismatch";
   };
 
   meta = with lib; {

pkgs/moira.nix

@@ -21,7 +21,7 @@ in stdenv.mkDerivation rec {
   sourceRoot = "source/moira";
 
   env = {
-    NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration -Wno-error=implicit-int -Wno-error=return-mismatch";
+    NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration";
   };
 
   meta = with lib; {