Version-check libtool correctly when doing an out-of-tree build

Version-check libtool correctly when doing an out-of-tree build

libtool is only created at the end of ./configure, so it doesn't make
sense to grep it in ./configure (the check would always fail the first
time). However, ltmain.sh is copied into the ${srcdir} by libtoolize and
should be safe to check at any time that configure can be run.

Signed-off-by: Simon McVittie <smcv@debian.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>