libXmu: Split into multiple outputs

We need to override BITMAPDIR, since the default:
    -DBITMAPDIR=\"$(includedir)/X11/bitmaps\"
would cause cycles between the outputs. Setting it to a nonexistent path
doesn't affect the code logic, since the corresponding single-output
directory $out/include/X11/bitmaps doesn't exist either.

(cherry picked from commit 15007c88108d9c6a3bc82b22eff53175bbddf75e)

pkgs/servers/x11/xorg/overrides.nix

@@ -185,6 +185,11 @@ in
     outputs = [ "dev" "out" "man" ];
   };
 
+  libXmu = attrs: attrs // {
+    outputs = [ "dev" "out" "doc" ];
+    buildFlags = ''BITMAP_DEFINES=-DBITMAPDIR=\"/no-such-path\"'';
+  };
+
   libXrandr = attrs: attrs // {
     outputs = [ "dev" "out" "man" ];
     preConfigure = setMalloc0ReturnsNullCrossCompiling;