Define _BSD_SOURCE in Stg.h

This means S_ISSOCK gets defined on Linux

includes/Stg.h

@@ -31,9 +31,13 @@
  */
 #ifndef IN_STG_CODE
 # define IN_STG_CODE 1
-# define _ISOC99_SOURCE
+
 // Turn on C99 for .hc code.  This gives us the INFINITY and NAN
 // constants from math.h, which we occasionally need to use in .hc (#1861)
+# define _ISOC99_SOURCE
+
+// Turning on _ISOC99_SOURCE means S_ISSOCK gets defined on Linux
+# define _BSD_SOURCE
 #endif
 
 #if IN_STG_CODE == 0