Provide easier configure options for GMP and readline

Provide easier configure options for GMP and readline

This patch uses the AC_LIB_LINKFLAGS macro, provided by Gnulib's
havelib module, to provide --with-gmp-prefix and
--with-readline-prefix configure options.  Many thanks to Bruno Haible
for suggesting and explaining this to me.

* configure.in (top level): Add AC_LIB_LINKFLAGS(gmp).

* guile-readline/configure.in (AC_CONFIG_AUX_DIR): Change to
  ../build-aux, to share the main build-aux directory and so avoid
  having to distribute multiple copies of config.rpath.
  (top level): Add AC_LIB_LINKFLAGS(readline).

* lib/Makefile.am, m4/gnulib-cache.m4: Regenerated by gnulib-tool for
  new import of the `havelib' module.