Perl 5 is required to compile Erlang from source. It's used (among other
things) to assemble BEAM opcodes, NIF jump tables and more. We now
assert perl exists on the PATH before we proceed to build Erlang or test
for Perl's version string.

We also now test for Java before we try to find its version. Java is not
required to compile Erlang though, so we don't output any warnings or
errors if its not found.

Unset ERL_TOP before kerl executes (#102)

Allow an underscore-prefixed alphanum suffix to the OTP version

Revert "Remove extraneous 's'"

This reverts commit aa237c72060e64a98b0e8a3b3be475387c73f53e.

This helps supporting custom forks of OTP e.g. basho/otp

Close #167