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)