Old MacOS SDKs require you include sys/socket.h before net/if.h (#673)

Old MacOS SDKs (like 10.8) require you include `sys/socket.h` before you include `net/if.h` as reported by MacPorts.  Using a new SDK but with setting the macos target version does not have the same issue.

Fixes Issue: #672 
Fix By: Brad House (@bradh352)

CMakeLists.txt

@@ -210,7 +210,12 @@ CHECK_INCLUDE_FILES (malloc.h              HAVE_MALLOC_H)
 CHECK_INCLUDE_FILES (memory.h              HAVE_MEMORY_H)
 CHECK_INCLUDE_FILES (netdb.h               HAVE_NETDB_H)
 CHECK_INCLUDE_FILES (netinet/in.h          HAVE_NETINET_IN_H)
-CHECK_INCLUDE_FILES (net/if.h              HAVE_NET_IF_H)
+# On old MacOS SDK versions, you must include sys/socket.h before net/if.h
+IF (HAVE_SYS_SOCKET_H)
+  CHECK_INCLUDE_FILES ("sys/socket.h;net/if.h"  HAVE_NET_IF_H)
+ELSE ()
+  CHECK_INCLUDE_FILES (net/if.h                 HAVE_NET_IF_H)
+ENDIF ()
 CHECK_INCLUDE_FILES (signal.h              HAVE_SIGNAL_H)
 CHECK_INCLUDE_FILES (socket.h              HAVE_SOCKET_H)
 CHECK_INCLUDE_FILES (stdbool.h             HAVE_STDBOOL_H)