Fix libtool initialization

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.

.gitlab-ci.yml

@@ -181,7 +181,6 @@ stages:
     --disable-maintainer-mode \
     --enable-developer \
     --with-libtool \
-    --disable-static \
     --with-cmocka \
     --with-libxml2 \
     --with-json-c \

configure.ac

@@ -110,7 +110,7 @@ AC_PROG_MAKE_SET
 # Checks for programs.
 m4_ifdef([AM_PROG_AR], [AM_PROG_AR]) # call AM_PROG_AR only if available
 
-AC_PROG_LIBTOOL
+AC_PATH_PROG([LIBTOOL], [libtool])
 AS_IF([test -z "$LIBTOOL"],
       [AC_MSG_ERROR([The libtool script could not be found.])])
 AC_PROG_INSTALL