Accommodate non-recursive Automake in a less hacky way.
* gnulib-tool: New option --automake-subdir.
(automake_subdir): New variable.
(func_emit_initmacro_end): Add a second argument. Use it to prefix each
object file name in *_LIBOBJS and *_LTLIBOBJS.
(func_emit_shellvars_init): New function.
(func_import): Add support for --automake-subdir. Invoke
prefix-gnulib-mk. Update calls to func_emit_initmacro_end. Call
func_emit_shellvars_init.
(func_create_testdir): Update calls to func_emit_initmacro_end. Call
func_emit_shellvars_init.
* m4/gnulib-tool.m4 (gl_AUTOMAKE_SUBDIR): New macro.
* m4/gnulib-common.m4 (gl_CONDITIONAL_HEADER): Use the value of the
gl_source_base_prefix variable.
* build-aux/prefix-gnulib-mk: New options --from-gnulib-tool, --prefix.
(contents_of_file): Renamed from contents.
(contents_of_stdin): New function.
(process): Inline and remove function.
* doc/gnulib-tool.texi (Non-recursive make): New section.