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.
