Fix Solaris compilation due to ctime_r() call.

Introduced in Redis 2.8.10 because of a change in Sentinel.
This closes issue #1837.

src/fmacros.h

@@ -46,6 +46,10 @@
 #define _XOPEN_SOURCE
 #endif
 
+#if defined(__sun)
+#define _POSIX_C_SOURCE 199506L
+#endif
+
 #define _LARGEFILE_SOURCE
 #define _FILE_OFFSET_BITS 64