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

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

From the origin commit message:
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.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>