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
 
 /*
--- a/perl_langinfo.h
+++ b/perl_langinfo.h
@@ -4,7 +4,11 @@
 #ifndef PERL_LANGINFO_H
 #define PERL_LANGINFO_H 1
 
+#ifdef USE_CROSS_COMPILE
+#include "xconfig.h"
+#else
 #include "config.h"
+#endif
 
 #if defined(HAS_NL_LANGINFO) && defined(I_LANGINFO)
 #   include <langinfo.h>