Makefile simplifications related to creating installation directories

A separate mkdir rule is not needed, since MKDIR_P is thread-safe.

* Makefile.in (install-arch-dep): Ensure bindir exists.
Drop mkdir dependency.
(install-arch-indep): Ensure docdir, infodir, mandir exist.
(install-leim): Drop mkdir dependency.
(mkdir): Remove most directories, now made in relevant rules.