Ensure visibility of needed Cygwin declarations

Dl_info requires __GNU_VISIBLE be defined; and,
pthread_setconcurrency requires  __XSI_VISIBLE >= 500.
Defining _GNU_SOURCE satifies both of the above needs.

include/wx/platform.h

@@ -280,6 +280,11 @@
 #    if defined(__INNOTEK_LIBC__)
         /* Ensure visibility of strnlen declaration */
 #        define _GNU_SOURCE
+#    endif
+
+#    if defined(__CYGWIN__)
+        /* Ensure visibility of Dl_info and pthread_setconcurrency declarations */
+#        define _GNU_SOURCE
 #    endif
 
     /* define __HPUX__ for HP-UX where standard macro is __hpux */