Optimize redundant 'mkdir -p .' invocations.

Reported by Paul Eggert.

* gnulib-tool (func_emit_lib_Makefile_am, func_emit_tests_Makefile_am):
Eliminate @NMD@ tokens.
* modules/alloca-opt (Makefile.am): Prefix the "$(MKDIR_P) '%reldir%'"
line with @NMD@.
* modules/argz (Makefile.am): Likewise.
* modules/assert-h (Makefile.am): Likewise.
* modules/byteswap (Makefile.am): Likewise.
* modules/configmake (Makefile.am): Likewise.
* modules/ctype (Makefile.am): Likewise.
* modules/dirent (Makefile.am): Likewise.
* modules/errno (Makefile.am): Likewise.
* modules/execinfo (Makefile.am): Likewise.
* modules/fcntl-h (Makefile.am): Likewise.
* modules/float (Makefile.am): Likewise.
* modules/fnmatch-h (Makefile.am): Likewise.
* modules/getopt-posix (Makefile.am): Likewise.
* modules/glob-h (Makefile.am): Likewise.
* modules/iconv-h (Makefile.am): Likewise.
* modules/ieee754-h (Makefile.am): Likewise.
* modules/inttypes-incomplete (Makefile.am): Likewise.
* modules/langinfo (Makefile.am): Likewise.
* modules/libtextstyle-optional (Makefile.am): Likewise.
* modules/limits-h (Makefile.am): Likewise.
* modules/locale (Makefile.am): Likewise.
* modules/malloc-h (Makefile.am): Likewise.
* modules/math (Makefile.am): Likewise.
* modules/monetary (Makefile.am): Likewise.
* modules/netdb (Makefile.am): Likewise.
* modules/openmp-init (Makefile.am): Likewise.
* modules/poll-h (Makefile.am): Likewise.
* modules/posix-shell (Makefile.am): Likewise.
* modules/pthread-h (Makefile.am): Likewise.
* modules/pty (Makefile.am): Likewise.
* modules/sched (Makefile.am): Likewise.
* modules/search (Makefile.am): Likewise.
* modules/signal-h (Makefile.am): Likewise.
* modules/sigsegv (Makefile.am): Likewise.
* modules/snippet/link-warning (Makefile.am): Likewise.
* modules/spawn (Makefile.am): Likewise.
* modules/stdalign (Makefile.am): Likewise.
* modules/stdarg (Makefile.am): Likewise.
* modules/stdbool (Makefile.am): Likewise.
* modules/stddef (Makefile.am): Likewise.
* modules/stdint (Makefile.am): Likewise.
* modules/stdio (Makefile.am): Likewise.
* modules/stdlib (Makefile.am): Likewise.
* modules/stdnoreturn (Makefile.am): Likewise.
* modules/string (Makefile.am): Likewise.
* modules/strings (Makefile.am): Likewise.
* modules/sysexits (Makefile.am): Likewise.
* modules/termios (Makefile.am): Likewise.
* modules/threads-h (Makefile.am): Likewise.
* modules/time (Makefile.am): Likewise.
* modules/uchar (Makefile.am): Likewise.
* modules/unicase/base (Makefile.am): Likewise.
* modules/uniconv/base (Makefile.am): Likewise.
* modules/unictype/base (Makefile.am): Likewise.
* modules/unigbrk/base (Makefile.am): Likewise.
* modules/unilbrk/base (Makefile.am): Likewise.
* modules/uniname/base (Makefile.am): Likewise.
* modules/uninorm/base (Makefile.am): Likewise.
* modules/unistd (Makefile.am): Likewise.
* modules/unistdio/base (Makefile.am): Likewise.
* modules/unistr/base (Makefile.am): Likewise.
* modules/unitypes (Makefile.am): Likewise.
* modules/uniwbrk/base (Makefile.am): Likewise.
* modules/uniwidth/base (Makefile.am): Likewise.
* modules/utime-h (Makefile.am): Likewise.
* modules/wchar (Makefile.am): Likewise.
* modules/wctype-h (Makefile.am): Likewise.

modules/string

@@ -27,7 +27,7 @@ BUILT_SOURCES += string.h
 # doesn't have one that works with the given compiler.
 string.h: string.in.h $(top_builddir)/config.status $(CXXDEFS_H) $(ARG_NONNULL_H) $(WARN_ON_USE_H)
 	$(AM_V_GEN)rm -f $@-t $@ && \
-	$(MKDIR_P) '%reldir%' && \
+@NMD@	$(MKDIR_P) '%reldir%' && \
 	{ echo '/* DO NOT EDIT! GENERATED AUTOMATICALLY! */' && \
 	  sed -e 's|@''GUARD_PREFIX''@|${gl_include_guard_prefix}|g' \
 	      -e 's|@''INCLUDE_NEXT''@|$(INCLUDE_NEXT)|g' \