perl-cross assumes the sources use xconfig.h with -DUSE_CROSS_COMPILE
With perl 5.20.0, it's no longer true. Regardless of what mainline perl
uses, let's stick with the old xconfig.h way for now.

--- a/perl.h
+++ b/perl.h
@@ -25,7 +25,11 @@
 #ifdef PERL_MICRO
 #   include "uconfig.h"
 #else
+#   ifndef USE_CROSS_COMPILE
 #   include "config.h"
+#   else
+#       include "xconfig.h"
+#   endif
 #endif
 
 /* this is used for functions which take a depth trailing