Scope gcc14 fixincludes override to Linux only

The gcc14 override with --disable-fixincludes was applied globally,
including aarch64-darwin where gfortran fails to bootstrap against
Apple SDK 14.4 (__FLT_EVAL_METHOD__ error in math.h). This caused
the Hydra evaluator to get stuck in an infinite retry loop for 12+
hours trying to build gfortran on darwin builders during IFD.

Scope the override to !isDarwin using optionalAttrs. The fixincludes
issue only affects Linux builders (Determinate Nix VM lacks /usr/include).

Author: angerman

flake.nix

@@ -49,7 +49,7 @@
            happy = final.haskell.lib.compose.overrideCabal (drv: {
              disallowGhcReference = false;
            }) prev.happy;
-
+         } // prev.lib.optionalAttrs (!prev.stdenv.hostPlatform.isDarwin) {
            # GCC 14's fixincludes step uses a configure-time substitution
            # @BUILD_SYSTEM_HEADER_DIR@ that bakes /usr/include into the
            # generated Makefile. On NixOS-based builders (Determinate Nix
@@ -59,6 +59,8 @@
            # --disable-fixincludes skips the fixincludes step entirely, which
            # is safe for Nix where all headers are store-managed. Already
            # used by nixpkgs' standalone libgcc build for the same reason.
+           # Only applied on Linux — on Darwin, GCC 14 bootstrap fails with
+           # __FLT_EVAL_METHOD__ errors against Apple SDK 14.4.
            gcc14 = prev.wrapCC (
              prev.gcc14.cc.overrideAttrs (old: {
                configureFlags = (old.configureFlags or []) ++ [ "--disable-fixincludes" ];