Hard coded Unix-style directory separator causes problems on Windows with nmake

Symptom:

<pre>        "C:\opt\perl-5.20.1\bin\perl.exe" mylib/gen-tests.perl lib/POE.pm
Can't open perl script "mylib": Permission denied
NMAKE : fatal error U1077: 'C:\opt\perl-5.20.1\bin\perl.exe' : return code '0xd'
Stop.</pre>

because perl ends up seeing `mylib` and `/gen-tests.perl` as two separate thingies.

Fix by using `File::Spec->catfile` to compose filenames.