build: build lib/ using non-recursive make

build: build lib/ using non-recursive make

* bootstrap.conf (gnulib_modules): Use the new module.
(bootstrap_post_import_hook): Invoke prefix-gnulib-mk.
* configure.ac (AC_CONFIG_FILES): Remove lib/Makefile.
* lib/Makefile.am: Renamed...
* lib/local.mk: ...to this.
* src/local.mk (CLEANFILES): Append, don't set.
(noinst_LIBRARIES): Likewise.
(AM_CPPFLAGS): Don't set this here.
* Makefile.am (AM_CPPFLAGS): Define here instead.
(noinst_LIBRARIES, CLEANFILES, MOSTLYCLEANDIRS, MOSTLYCLEANFILES):
Initialize here, so we can append to them from each included local.mk
(SUBDIRS): Remove "lib".