TCP_NOTSENT_LOWAT older kernel compatibility

Motiviation:
TCP_NOTSENT_LOWAT is only supported in linux kernel 3.12 or newer. The addition of this socket option prevents older kernels from building.

Modifications:
- Conditionally define TCP_NOTSENT_LOWAT if it is not defined

Result:
Kernels older than 3.12 can still compile the EPOLL module.

transport-native-epoll/src/main/c/io_netty_channel_epoll_Native.c

@@ -35,6 +35,11 @@
 #include "io_netty_channel_epoll_Native.h"
 #include "exception_helper.h"
 
+// TCP_NOTSENT_LOWAT is defined in linux 3.12. We define this here so older kernels can compile.
+#ifndef TCP_NOTSENT_LOWAT
+#define TCP_NOTSENT_LOWAT 25
+#endif
+
 /**
  * On older Linux kernels, epoll can't handle timeout
  * values bigger than (LONG_MAX - 999ULL)/HZ.