configure.ac: Use `GUILE_PKG'
m4/{guile-2.0,pkg}.m4: Add to the repository

* Makefile.am (SUBDIRS): Add `m4'.
* configure.ac (AC_CONFIG_MACRO_DIR): Set to `m4'.
  (AC_CONFIG_FILES): Add `m4/Makefile'.
* m4/guile-2.0.m4, m4/pkg.m4, m4/Makefile.am: Add to the repository.