Only use libC is not using the GNU c++ compiler.

configure.in

@@ -263,11 +263,13 @@ AC_CHECK_LIB(c_r, gethostbyname_r)
 AC_CHECK_LIB(c, strcpy)
 AC_CHECK_LIB(m, atan)
 AC_CHECK_LIB(dl, dlopen)
+if [ test -z "$GNU_CXX" ]; then
+    AC_CHECK_LIB(C, demangle)
+fi
 AC_CHECK_LIB(resolv, res_gethostbyaddr)
 AC_CHECK_LIB(socket, getnetbyaddr)
 AC_CHECK_LIB(util, login)
 AC_CHECK_LIB(nsl, xdr_string)
-AC_CHECK_LIB(C, demangle)
 AC_CHECK_LIB(elf, elf_rawdata)
 AC_CHECK_LIB(intl, textdomain)
 AC_CHECK_LIB(posix4, shm_open)