Test the lock-fork only if $$ is writeable

On Perl 5.10 (RH6) $$ is read-only and the test crashes.  I have tested
this already on SL5, and the bug shows up on my Fedora before the patch
and is fixed after the patch.

And it's a corner case for a low-impact bug.
Merge pull request #8 from piojo/test-rh

Test the lock-fork only if $$ is writeable