bootstrap: make sure patch is available to bootstrap if necessary.

bootstrap: make sure patch is available to bootstrap if necessary.

Make sure we're not lacking any functionality provided by the
incumbent gnulib bootstrap script, and where it's easy to do so
(like it is here), provide better functionality!
* bootstrap (require_patch): Make sure that PATCH expands to the
best `patch' command available on the system, honouring
overrides in the users environment.
(require_patch_buildreq): Automatically add a requirement for a
patch command to buildreq when there are diff files in
$local_gl_dir.
(func_require_buildtools_uptodate): Call require_patch_buildreq
before testing build tool versions.
(func_gnulib_tool_copy_file): Make sure PATCH is set before using
gnulib-tool to copy files.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>