libstc++ isn't in staging anymore, so we must use g++ for linking

--- /tmp/configure.ac	2008-03-31 11:39:13.130631737 +0200
+++ abiword-plugins-2.6.0/configure.ac	2008-03-31 11:39:22.754586487 +0200
@@ -14,6 +14,7 @@
 AC_PROG_INSTALL
 AC_PROG_LN_S
 AC_PROG_MAKE_SET
+AC_LANG_CPLUSPLUS
 
 AM_INIT_AUTOMAKE