Define HAVE_LIBDL, if dlopen() was found.

It was tempting to do a complete s/HAVE_LIBDL/HAVE_DLOPEN/ over the
whole code base.  I defer that to a later point in time for now.

configure.in

@@ -280,6 +280,9 @@ PHP_CHECK_FUNC(gethostbyaddr, nsl)
 PHP_CHECK_FUNC(yp_get_default_domain, nsl)
 
 PHP_CHECK_FUNC(dlopen, dl)
+if test "$ac_cv_func_dlopen" = "yes"; then
+  AC_DEFINE(HAVE_LIBDL, 1, [ ])
+fi
 AC_CHECK_LIB(m, sin)
 
 dnl Check for resolver routines.