Revert gcc14 --disable-fixincludes override

The gcc14 override caused a full x86_64-linux toolchain rebuild,
including glibc. The rebuilt glibc fails on Determinate Nix's Linux
VM (darwin builders) with "undefined symbol: __libc_start_main",
breaking 178+ x86_64-linux Haskell builds — far worse than the 32
Windows cross-builds the override was meant to fix.

The fixincludes issue needs a narrower solution that doesn't
invalidate all x86_64-linux derivation hashes.

Author: angerman

flake.nix

@@ -49,24 +49,6 @@
            happy = final.haskell.lib.compose.overrideCabal (drv: {
              disallowGhcReference = false;
            }) prev.happy;
-         } // prev.lib.optionalAttrs (prev.stdenv.hostPlatform.system == "x86_64-linux") {
-           # 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
-           # Linux VM), /usr/include doesn't exist, causing x86_64-linux
-           # GCC builds to fail — cascading to all Windows cross-builds.
-           # --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.
-           # Scoped to x86_64-linux only: on Darwin, gfortran bootstrap fails
-           # (Apple SDK __FLT_EVAL_METHOD__); on aarch64-linux, changing gcc14
-           # cascades through the entire IFD chain, and linux-headers can't be
-           # rebuilt on darwin builders (case-insensitive APFS).
-           gcc14 = prev.wrapCC (
-             prev.gcc14.cc.overrideAttrs (old: {
-               configureFlags = (old.configureFlags or []) ++ [ "--disable-fixincludes" ];
-             })
-           );
          });
 
          cddl-tools = (final: prev: {