Fixups for GNU Make switchover

Fixups for GNU Make switchover

This fixes some minor problems introduced in the recent switch to GNU
Make, discovered by further testing.  Without some of these changes
'make -j' would sometimes have race conditions caused by missing
dependencies.  (Bug#26100)
* .gitignore: Remove src/stamp-h.in, src/stamp-h1.
* Makefile.in ($(MAKEFILE_NAME)): Depend on configure, not
src/config.in, since the former's timestamp now represents
the latter's.
($(srcdir)/configure): Use plain ./autogen.sh, for consistency
with other autogen.sh invocations.
($(srcdir)/src/stamp-h.in):
Remove rule, as this file is no longer created.
* Makefile.in (top_distclean):
* src/Makefile.in (bootstrap-clean):
No need to remove stamp-h1, as that was an Automake byproduct
and Automake is no longer in use.
* lib/Makefile.in, src/Makefile.in:
(AUTOCONF_INPUTS, $(top_srcdir)/configure): Remove.
(../config.status, Makefile): Simplify by limiting dependencies
to files we care about and files in the repository, and by
using just one file to represent the timestamps on multiple
targets updated by the same rule.
* autogen.sh: Do not create or use src/stamp-h.in.
Instead, have 'find' test the two output files directly.