configure: warn if HTTP upgrade is disabled

configure.ac

@@ -6064,6 +6064,7 @@ AS_IF([test "x$enable_bauth" != "xyes" || \
    test "x${USE_THREADS}" = "xnone" || \
    test "x$enable_httpupgrade" != "xyes" || \
    test "x$enable_cookie" != "xyes" || \
+   test "x$enable_httpupgrade" != "xyes" || \
    test "x$enable_postprocessor" != "xyes"],
    [AC_MSG_WARN([This will be a custom build with missing symbols. Do NOT use this build in a distribution. Building with these kinds of configure options is only for custom builds for embedded systems.])]
 )