Convert lib/Makefile.am into lib/local.mk.

The real problem is rather gnulib.mk, which itself is extracted from a
Makefile.am that gnulib expects to the "recursive".  The tool
prefix-gnulib-mk converts such a gnulib.mk to be non-recursive.  Also,
some AC_SUBST variables need to be adjusted.

	* etc/prefix-gnulib-mk: New.
	* bootstrap (slurp): Use it to convert further gnulib.mk.
	* lib/Makefile.am: Rename as...
	* lib/local.mk: this.
	Adjust to be prefixed.
	* Makefile.am, configure.ac: Adjust.
	* src/local.mk (AM_CPPFLAGS): Extend it, don't define it.

configure.ac

@@ -80,7 +80,7 @@ AC_ARG_ENABLE([yacc],
 case $enable_yacc in
 yes)
   YACC_SCRIPT=src/yacc
-  YACC_LIBRARY=liby.a;;
+  YACC_LIBRARY=lib/liby.a;;
 *)
   YACC_SCRIPT=
   YACC_LIBRARY=;;
@@ -148,9 +148,14 @@ AC_SUBST([O0CXXFLAGS], [`echo $CXXFLAGS | sed 's/-O[[0-9]] *//'`])
 gt_JAVACOMP([1.3], [1.4])
 gt_JAVAEXEC
 
+AC_SUBST([gl_PREFIXED_LIBOBJS],
+         [$(echo "$gl_LIBOBJS" | sed -e 's, , lib/,g')])
+for ac_var in ERRNO_H GETOPT_H INTTYPES_H STDBOOL_H STDINT_H WCHAR_H WCTYPE_H
+do
+  eval "$ac_var=lib/\$$ac_var"
+done
 AC_CONFIG_FILES([Makefile
 		 po/Makefile.in
 		 examples/calc++/Makefile
-		 lib/Makefile
 		 doc/yacc.1])
 AC_OUTPUT