Define XML_STATIC in Expat header rather than Expat projects.

This defines XML_STATIC unconditionally in Expat sources themselves as the built-in Expat is always used as a static library, so this solution is simpler and more portable.

src/expat/lib/expat_external.h

@@ -5,6 +5,13 @@
 #ifndef Expat_External_INCLUDED
 #define Expat_External_INCLUDED 1
 
+/* wxWidgets modification: always predefine XML_STATIC as we always use
+   Expat as a static library.
+*/
+#ifndef XML_STATIC
+#define XML_STATIC
+#endif
+
 /* External API definitions */
 
 #if defined(_MSC_EXTENSIONS) && !defined(__BEOS__) && !defined(__CYGWIN__)