tools: fix libtool version check for Debian

On Debian, the libtool version is prefixed by "Debian-":

$ libtool --version | head -1
libtool (GNU libtool) 2.5.4 Debian-2.5.4-4

As a consequence, the extracted version is incorrect:

$ libtool --version | head -1 | sed s/'.* '//
Debian-2.5.4-4

causing the bootstrap script to fail:

./bootstrap: 31: [: Illegal number: Debian-2
./bootstrap: 32: [: Illegal number: Debian-2
./bootstrap: 33: [: Illegal number: Debian-2

Extract the first pattern matching NUM.NUM or NUM.NUM.NUM.