The LT_INIT() call in configure.ac is effectively a no-op because it is
preceded by a call to AC_PROG_LIBTOOL(), which is the previous name of
LT_INIT() used in older libtool versions.  Replace AC_PROG_LIBTOOL()
with AC_PATH_PROG() to look for libtool in PATH without initializing it,
which is the originally intended behavior.

Without this change, --enable-static is used by default, which causes a
plain ./configure invocation to fail because static linking is now
disallowed.  Drop --disable-static from the ./configure invocations used
in GitLab CI to test this scenario continuously.