configure: Explicitly require Libtool 2.2+.

configure.ac

@@ -33,8 +33,8 @@ AC_PROG_SED
 AC_PROG_LN_S
 AC_PROG_F77
 
-AC_LIBTOOL_WIN32_DLL
-AC_PROG_LIBTOOL
+LT_PREREQ([2.2])
+LT_INIT([win32-dll])
 
 AC_PROG_INSTALL
 AC_PROG_MKDIR_P