freetype, pixman: restore flatten include hack for now, but better

pkgs/build-support/setup-hooks/flatten-include-hack/default.nix

@@ -0,0 +1,2 @@
+{ makeSetupHook }:
+makeSetupHook { name = "flatten-include-hack-hook"; } ./flatten-include-hack-hook.sh

pkgs/build-support/setup-hooks/flatten-include-hack/flatten-include-hack-hook.sh

@@ -0,0 +1,16 @@
+# shellcheck shell=bash
+
+# This is a horrible hack. You should not use this.
+
+flattenIncludes() {
+    (
+        cd "${!outputInclude}/include" || exit
+        for file in */*; do
+            target=$(basename "$file")
+            echo "[HACK] Symlinking include $file to flattened path $target..."
+            ln -s "$file" "$target"
+        done
+    )
+}
+
+preFixupHooks+=(flattenIncludes)

pkgs/development/libraries/freetype/default.nix

@@ -33,6 +33,7 @@
 , texmacs
 , ttfautohint
 , testers
+, __flattenIncludeHackHook
 }:
 
 
@@ -48,7 +49,7 @@ stdenv.mkDerivation (finalAttrs: {
   propagatedBuildInputs = [ zlib bzip2 brotli libpng ]; # needed when linking against freetype
 
   # dependence on harfbuzz is looser than the reverse dependence
-  nativeBuildInputs = [ pkg-config which ]
+  nativeBuildInputs = [ pkg-config which __flattenIncludeHackHook ]
     ++ lib.optional (!stdenv.hostPlatform.isWindows) makeWrapper
     # FreeType requires GNU Make, which is not part of stdenv on FreeBSD.
     ++ lib.optional (!stdenv.hostPlatform.isLinux) gnumake;

pkgs/development/libraries/pixman/default.nix

@@ -18,6 +18,8 @@
 
 , gitUpdater
 , testers
+
+, __flattenIncludeHackHook
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -40,7 +42,7 @@ stdenv.mkDerivation (finalAttrs: {
 
   separateDebugInfo = !stdenv.hostPlatform.isStatic;
 
-  nativeBuildInputs = [ meson ninja pkg-config ];
+  nativeBuildInputs = [ meson ninja pkg-config __flattenIncludeHackHook ];
 
   buildInputs = [ libpng ];
 

pkgs/top-level/all-packages.nix

@@ -161,6 +161,8 @@ with pkgs;
 
   ### BUILD SUPPORT
 
+  __flattenIncludeHackHook = callPackage ../build-support/setup-hooks/flatten-include-hack { };
+
   autoreconfHook = callPackage (
     { makeSetupHook, autoconf, automake, gettext, libtool }:
     makeSetupHook {