XSConfig compatibility

With XSConfig with p5p perl, or cperl, Test::Harness fails in
t/nofork-mux.t because of reliance on internal implementation of Config.pm.

Without using any external libraries or modules, fake not having fork by
putting a shim between the XS FETCH sub/method and the Config class that
traps and changes 'd_fork' key. Sample of error below that is fixed with
this patch.

C:\cpan\testharness>prove -v t/nofork-mux.t
t/nofork-mux.t .. Not a HASH reference at t/lib/NoFork.pm line 7.
Compilation failed in require at t/nofork-mux.t line 10.
BEGIN failed--compilation aborted at t/nofork-mux.t line 10.
Dubious, test returned 255 (wstat 65280, 0xff00)
No subtests run

Test Summary Report
-------------------
t/nofork-mux.t (Wstat: 65280 Tests: 0 Failed: 0)
  Non-zero exit status: 255
  Parse errors: No plan found in TAP output
Files=1, Tests=0,  0 wallclock secs ( 0.11 usr +  0.00 sys =  0.11 CPU)
Result: FAIL

Original author of this code is Reini Urban, in cperl commit
42cac300080de7d3a9df8eed040750ec9583de20 "Test-Harness: fix 2 tests"
on "Date: 9/28/2015 10:59:36 AM". The code was submitted upstream in Apr
2017 in Perl-Toolchain-Gang#63 but
that PR stalled. I've extracted the minimum code needed to get T::H to pass
with XSConfig with p5p perl and placed it in this patch so that the code
can be published sooner.

t/lib/NoFork.pm

@@ -4,7 +4,21 @@ BEGIN {
     *CORE::GLOBAL::fork = sub { die "you should not fork" };
 }
 use Config;
-tied(%Config)->{d_fork} = 0;    # blatant lie
+if ($Config::Config{d_fork}) {
+  if (exists &Config::KEYS) {     # compiled Config
+    *Config_FETCHorig = \&Config::FETCH;
+    no warnings 'redefine';
+    *Config::FETCH = sub {
+      if ($_[0] and $_[1] eq 'd_fork') {
+        return 0;
+      } else {
+        return Config_FETCHorig(@_);
+      }
+    }
+  } else {
+    tied(%Config)->{d_fork} = 0;    # uncompiled Config
+  }
+}
 
 =begin TEST