Fix HAVE_ORTP_LOG_DOMAIN detection on FreeBSD

We need to use the include path[s] resolved by pkg-config for
ortp before trying to compile our test program.  Without this
patch, compilation will fail e.g. on FreeBSD 11.

Change-Id: I0c95e37cc449297389bf31dd9d5dcaa922d75d0c
diff --git a/configure.ac b/configure.ac
index 43266be..9b2f189 100644
--- a/configure.ac
+++ b/configure.ac
@@ -59,6 +59,8 @@
 	CPPFLAGS+=" -fsanitize=address -fsanitize=undefined"
 fi
 
+_cflags_save=$CFLAGS
+CFLAGS="$CFLAGS $ORTP_CFLAGS"
 AC_COMPILE_IFELSE(
 	[AC_LANG_PROGRAM(
 		[[#include <ortp/ortp.h>]],
@@ -68,6 +70,7 @@
 		[ortp_set_log_level_mask requires domain parameter])],
 	[AC_DEFINE([HAVE_ORTP_LOG_DOMAIN], [0],
 		[ortp_set_log_level_mask has no domain parameter])])
+CFLAGS=$_cflags_save
 
 AC_OUTPUT(
 	libosmoabis.pc