maint: bootstrap: run autopoint and libtoolize *before* gnulib-tool

maint: bootstrap: run autopoint and libtoolize *before* gnulib-tool

* bootstrap (AUTOPOINT, AUTORECONF): Factor out definitions.
Run autopoint and libtoolize *before* gnulib-tool.
After it, run an abbreviated autoreconf, rather than a loop around
all tools.
* bootstrap.conf (gnulib_tool_option_extras): Add both --symlink
and --makefile-name=gnulib.mk.  Remove stray use of $bt.
* lib/Makefile.am: Initialize all of the following so that
generated code in gnulib.mk may use += to append to those variables:
AM_CFLAGS, AM_CPPFLAGS, BUILT_SOURCES, CLEANFILES, EXTRA_DIST,
MAINTAINERCLEANFILES, MOSTLYCLEANDIRS, MOSTLYCLEANFILES, SUFFIXES,
noinst_LIBRARIES.