* Makefile.am (maintainer-check): Check for mkdir_p.

* Makefile.am (maintainer-check): Check for mkdir_p.
* automake.texi (Obsolete macros): Document AM_PROG_MKDIR_P.
* lib/am/data.am, lib/am/distdir.am, lib/am/install.am,
lib/am/java.am, lib/am/libs.am, lib/am/lisp.am, lib/am/ltlib.am,
lib/am/mans.am, lib/am/progs.am, lib/am/python.am,
lib/am/scripts.am, lib/am/texinfos.am: Use MKDIR_P instead of mkdir_p.
* m4/mkdirp.m4 (AM_PROG_MKDIR_P): Define mkdir_p using $MKDIR_P, not
as '$(MKDIR_P)', otherwise it will break `Makefile.in's that use
mkdir_p without defining MKDIR_P.
* tests/distdir.test, tests/instman.test, tests/txinfo21.test:
Adjust.