WIP --cross=path/config.sh support

See GH #428

Makefile.PL

@@ -97,7 +97,7 @@ sub write_b_c_config {
     print PH "# broken or patched upstream 5.22 ByteLoader. undef for yet undecided\n";
     $have_byteloader = 0 if $Config{usecperl}; # have it, but still broken
     print PH "\$have_byteloader = $have_byteloader;\n";
-    print PH "# cperl 5.22.2:\n";
+    print PH "# since cperl 5.22.2:\n";
     print PH "\$have_op_rettype = $have_op_rettype;\n";
     print PH "\$have_HEK_STATIC = $have_HEK_STATIC;\n";
     print PH "# use extra compiler flags, after ccopts, resp. ldopts\n";
@@ -120,7 +120,10 @@ sub write_b_c_config {
 	          static_ext usecperl usedl useithreads uselongdouble usemultiplicity
 	          usemymalloc uvuformat))
     {
-        if ($CORE) {
+        if ($cross) { # -cross=path/config.sh values
+            my $v = cross_config($cross, $s);
+        }
+        elsif ($CORE) {
             my $v = $Config::Config{$s};
             if (!defined $v) {
                 $v = 'undef';