Narrow gcc14 fixincludes override to isLinux only

Using !isDarwin would also rebuild gcc14 for Windows (mingw32) targets
unnecessarily. The fixincludes /usr/include issue only affects Linux
builders, so scope to isLinux to avoid touching darwin and windows
gcc14 derivations.

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) {
+         } // prev.lib.optionalAttrs prev.stdenv.hostPlatform.isLinux {
            # 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
@@ -59,8 +59,9 @@
            # --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.
+           # Only applied on Linux — avoids unnecessary gcc14 rebuilds on
+           # Darwin (where gfortran fails against Apple SDK 14.4) and
+           # Windows (where the mingw cross-GCC doesn't have this issue).
            gcc14 = prev.wrapCC (
              prev.gcc14.cc.overrideAttrs (old: {
                configureFlags = (old.configureFlags or []) ++ [ "--disable-fixincludes" ];