Check if Makefile.PL has the right pre-requisites

The prereq_ok() function examines the modules it finds in blib/lib/,
blib/script, and the test files it finds in t/ (and test.pl). It figures
out which modules they use, skips the modules that are in the Perl core,
and compares the remaining list of modules to those in the PREREQ_PM
section of Makefile.PL.