include linux/tcp.h on Linux pre-glibc 2.17

Bazel 0.24.0 upgraded grpc from 1.13.0 to 1.18.0, and the latter makes use of
the TCP_USER_TIMEOUT socket option.  Problem:  grpc conditions the option on
the Linux kernel version but sources the option from glibc's `netinet/tcp.h`.
glibc != Linux kernel, and that option wasn't imported to glibc until 2.17.

We can't just build Bazel with glibc 2.17 because we still have to support
CentOS 6 dev nodes which ship with glibc 2.12.  So instead this change
tweaks the grpc headers to conditionally include <linux/tcp.h> instead of
<netinet/tcp.h>.

Resolves https://github.com/grpc/grpc/issues/18728.

src/core/lib/iomgr/port.h

@@ -88,6 +88,15 @@
 #ifdef LINUX_VERSION_CODE
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 37)
 #define GRPC_HAVE_TCP_USER_TIMEOUT
+#ifdef __GLIBC_PREREQ
+#if !(__GLIBC_PREREQ(2, 17))
+/*
+ * TCP_USER_TIMEOUT wasn't imported to glibc until 2.17. Use Linux system
+ * header instead.
+ */
+#define GRPC_LINUX_TCP_H 1
+#endif /* __GLIBC_PREREQ(2, 17) */
+#endif /* ifdef __GLIBC_PREREQ */
 #endif /* LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 37) */
 #endif /* LINUX_VERSION_CODE */
 #ifndef __GLIBC__

src/core/lib/iomgr/socket_utils_common_posix.cc

@@ -30,7 +30,11 @@
 #include <fcntl.h>
 #include <limits.h>
 #include <netinet/in.h>
+#ifdef GRPC_LINUX_TCP_H
+#include <linux/tcp.h>
+#else
 #include <netinet/tcp.h>
+#endif
 #include <stdio.h>
 #include <string.h>
 #include <sys/socket.h>