Make merging from gnulib a script, not a makefile action.

Make merging from gnulib a script, not a makefile action.

Putting it in a makefile has some problems with reflection, as
merging from gnulib updates 'configure', which can update the makefile.
Putting it in a standalone script breaks this loop.
* Makefile.in (gnulib_srcdir, $(gnulib_srcdir), DOS_gnulib_comp.m4)
(GNULIB_MODULES, GNULIB_TOOL_FLAGS, sync-from-gnulib):
Remove, moving the actions to the script admin/merge-gnulib.
* admin/merge-gnulib: New script, with actions moved here from
../Makefile.in.