Let configure test for a suitable mkdir -p

Let configure test for a suitable mkdir -p

* configure.in (AC_PROG_MKDIR_P): Call it, to set MKDIR_P.
(MKDEPDIR): Use $MKDIR_P.

* Makefile.in (MKDIR_P): New, set by configure.
(mkdir): Use $MKDIR_P.

* doc/emacs/Makefile.in (MKDIR_P): New, set by configure.
(mkinfodir): Use $MKDIR_P.

* doc/lispintro/Makefile.in (MKDIR_P): New, set by configure.
(mkinfodir): Use $MKDIR_P.

* doc/lispref/Makefile.in (MKDIR_P): New, set by configure.
(mkinfodir): Use $MKDIR_P.

* doc/misc/Makefile.in (MKDIR_P): New, set by configure.
(mkinfodir): Use $MKDIR_P.

* src/Makefile.in (MKDIR_P): New, set by configure.

* src/ns.mk (${ns_appdir}, ${ns_appbindir}Emacs): Use $MKDIR_P.