libtool: don't execute automake and autoconf on every invocation.

libtool: don't execute automake and autoconf on every invocation.

Regression from 2.4.2 was causing noticable slow-down in builds
that call libtool many times.
* build-aux/ltmain.in (func_help): Override func_help() from
gl/build-aux/options-parser to only run automake --version and
autoconf --version when libtool --help is executed on the command
line.
* NO-THANKS: Add Robert Yang.
Reported by Robert Yang

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