configure.ac: Fix condition check for RTP_SIGNAL_PTR_CAST define

While building libosmo-netif on a system with an old ortp (0.22), it
was seen that warning related to RTP_SIGNAL_PTR_CAST not being correctly
selected were being printed. The compilation didn't fail there as
expected because the issue was being handled as a warning instead of an
error, so the void* ptr case was always being selected.

Change-Id: Ib5e7569e68fdb921c63e0c12e20e73aafbebda46
diff --git a/configure.ac b/configure.ac
index ce4bf39..05fbb12 100644
--- a/configure.ac
+++ b/configure.ac
@@ -110,7 +110,7 @@
 CFLAGS=$_cflags_save
 
 _cflags_save=$CFLAGS
-CFLAGS="$CFLAGS $ORTP_CFLAGS"
+CFLAGS="$CFLAGS $ORTP_CFLAGS -Werror"
 AC_COMPILE_IFELSE(
 	[AC_LANG_PROGRAM(
 		[[#include <ortp/ortp.h>