make_ext.pl: Phase out the use of Cross.pm

make_ext.pl: Phase out the use of Cross.pm

In the old cross-compilation model, lib/ for the target would
end up in xlib/; What Cross.pm did back then was change @INC
around a bit to have miniperl point to xlib; Additionally, it
was used to identify cross-compilation builds, and in make_ext.pl
to detect whenever a Makefile was from the host's build.

There is no longer any need for the first, and the second is now
a simple check for $Config{usecrosscompile}, it's still
possible for the hosts' Makefiles to be there, particularly
if the host was not compiled elsewhere using -Dmksymlinks,
like in Windows when targetting WinCE.

This commit changes make_ext.pl for cross-compiling builds
to only delete Makefiles when their CC != $Config{cc}.