Convert from using mkinstalldirs to $(MKDIR_P).

Convert from using mkinstalldirs to $(MKDIR_P).

The automake mkinstalldirs script is now deprecated. "install-sh -d"
can be used in place.  Configure.ac already includes AC_PROG_INSTALL,
which will ensure install-sh is bundled.  Add AC_PROG_MKDIR_P, which
will set $(MKDIR_P) to either a thread-safe "mkdir -p" or will fall back
to using install-sh.