Don't declare _GNU_SOURCE on Windows

Doing so will cause `network` to have an undeclared dependency against the
`mingwex` library on Windows, which can cause issues with GHC's runtime linker.
(See https://gitlab.haskell.org/ghc/ghc/-/issues/23309 for the full story.)
Thankfully, there is no particular need to define `_GNU_SOURCE` on Windows in
the first place, so I have guarded its definition with CPP.

Fixes #551.

Author: RyanGlScott

configure.ac

@@ -4,7 +4,9 @@ AC_INIT([Haskell network package],
         [network])
 
 dnl See also HsNet.h
-ac_includes_default="#define _GNU_SOURCE 1  /* for struct ucred on Linux */
+ac_includes_default="#ifndef _WIN32
+# define _GNU_SOURCE 1  /* for struct ucred on Linux */
+#endif
 $ac_includes_default
 
 #ifdef _WIN32

include/HsNet.h

@@ -19,7 +19,9 @@
 # endif
 #endif
 
-#define _GNU_SOURCE 1 /* for struct ucred on Linux */
+#ifndef _WIN32
+# define _GNU_SOURCE 1 /* for struct ucred on Linux */
+#endif
 #define __APPLE_USE_RFC_3542 1 /* for IPV6_RECVPKTINFO */
 
 #ifdef _WIN32