Add macOS-specific TCP variables

(from github.com/Loxodromics/ludimus 5c2e40f)
SOL_TCP and TCP_KEEPIDLE are defined in Linux but not MacOS
If they are undefined, replace SOL_TCP by IPPROT0_TCP and
replace TCP_KEEPIDLE by TCP_KEEPALIVE

lib/raop_rtp_mirror.c

@@ -30,6 +30,13 @@
 #include "mirror_buffer.h"
 #include "stream.h"
 
+/* for MacOS, where SOL_TCP and TCP_KEEPIDLE are not defined */
+#if !defined(SOL_TCP) && defined(IPPROTO_TCP)
+#define SOL_TCP IPPROTO_TCP
+#endif
+#if !defined(TCP_KEEPIDLE) && defined(TCP_KEEPALIVE)
+#define TCP_KEEPIDLE TCP_KEEPALIVE
+#endif
 
 struct h264codec_s {
     unsigned char compatibility;