Test for cygwin to avoid Linux specific code

Compile failed on Cygwin looking for Linux specific header files.  Add a
preprocessor test for cygwin to only compile Windows compatible code.

winradio/g313.c

@@ -28,6 +28,7 @@
 #define G313_MODES (RIG_MODE_NONE)
 
 #ifdef _WIN32
+#ifdef __CYGWIN__
 
 #ifdef HAVE_WINDOWS_H
 #include <windows.h>
@@ -642,7 +643,7 @@ int g313_get_conf(RIG *rig, token_t token, char *val)
 	return RIG_OK;
 }
 
-/* end _WIN32 */
+/* end _WIN32 and __CYGWIN__ */
 #else
 
 /* linux, maybe other posix */
@@ -1148,4 +1149,5 @@ const struct rig_caps g313_caps =
   .get_info =      g313_get_info,
 };
 
+#endif	/* not __CYGWIN__ */
 #endif	/* not _WIN32 */

winradio/linradio/wrg313api.c

@@ -1,4 +1,5 @@
 #ifndef _WIN32
+#ifndef __CYGWIN__
 
 #include <dlfcn.h>
 #include <stdio.h>
@@ -168,4 +169,5 @@ void UninitAPI(void)
     }
 }
 
-#endif
+#endif	/* not __CYGWIN__ */
+#endif	/* not _WIN32 */

winradio/winradio.c

@@ -246,10 +246,18 @@ DECLARE_INITRIG_BACKEND(winradio)
 
 	/* Receivers with DLL only available under Windows */
 #ifdef _WIN32
+#ifdef __CYGWIN__
 	rig_register(&g303_caps);
 	rig_register(&g305_caps);
 #endif
+#endif
+
+	/* Not available on MS Windows */
+#ifndef _WIN32
+#ifndef __CYGWIN__
 	rig_register(&g313_caps);
+#endif
+#endif
 
 	return RIG_OK;
 }