bisect-runner.pl now builds test_prep on OS X back to 5.001n

bisect-runner.pl now builds test_prep on OS X back to 5.001n

bisect.pl is now suitable for general use on (at least) Linux, FreeBSD and
OS X. [Tested on Snow Leopard on a case-sensitive file system. The latter is
a requirement for some older intermediate revisions of perl]

bisect-runner.pl now sets $ENV{$Config{ldlibpthname}} before running the
supplied test case, which is necessary when perl is built with
useshrplib='true'

The historical Darwin hints require some tweaking for cflags and ldflags.
Adding the Darwin hints and dl_dyld.xs with minimal patching is sufficient
to build perl and all extensions back to 5.001n, allowing Darwin systems to
be used for general bisecting, not just Darwin specific issues.