compat: provide va_copy for old gcc versions.

Since we have this compat/va_copy.h header already we might just as well make
use of it for more than one compiler.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

compat/va_copy.h

@@ -24,3 +24,6 @@
 #if !defined(va_copy) && defined(_MSC_VER)
 #define va_copy(dst, src) ((dst) = (src))
 #endif
+#if !defined(va_copy) && defined(__GNUC__) && __GNUC__ < 3
+#define va_copy(dst, src) __va_copy(dst, src)
+#endif