gitlab CI: ensure libtool is installed in build container
Fixes build failure:
autoreconf: running: /usr/bin/autoconf --force
configure.ac:43: error: possibly undefined macro: AC_DISABLE_STATIC
If this token and others are legitimate, please use m4_pattern_allow.
See the Autoconf documentation.
configure.ac:44: error: possibly undefined macro: AC_PROG_LIBTOOL
autoreconf: error: /usr/bin/autoconf failed with exit status: 1