build: Use Automake in lib/.

* autopull.sh: Copy also m4/init-package-version.m4.
* autogen.sh: Mention the requirement for Automake.
* Makefile.devel (lib/aclocal.m4, lib/config.h.in, lib/Makefile.in): New
rules.
(lib/configure): Use lib/aclocal.m4 instead of the top-level aclocal.m4.
(src/config.h.in): Fix rule.
(totally-clean): Remove the new generated files.
* lib/configure.ac: Require Autoconf >= 2.70. Change AC_INIT invocation.
Invoke AC_CONFIG_MACRO_DIRS, gl_INIT_PACKAGE_VERSION, AM_INIT_AUTOMAKE,
AC_CONFIG_HEADERS. Don't invoke AC_MAKE_SET, AC_PROG_CPP,
AC_PROG_CXXCPP, AC_PROG_INSTALL.
* lib/Makefile.am: New file.
* lib/Makefile.in: Remove file.
* Makefile.in (IMPORTED_FILES): Remove lib/filename.h.

build-aux/.gitignore

@@ -3,4 +3,7 @@
 /mkinstalldirs
 /compile
 /ar-lib
+# Files brought in by automake:
+/depcomp
+/missing