Add some variable definitions to Makefile

This way, we can easily override the tools to use via

    make NQP=path/to/nqp
Add helper function for path lists
Add support for cross-compilation from Cygwin to Windows

The environment variable CYGCROSS needs to be set

There are currently test failures in

    t/qast/qast_process.t
    t/qast/qast_stat.t

which might be artifacts of the Frankensteinian setup;
all other tests pass
[BUGFIX] t/qast/qast_process.t failed due to a typo
Merge pull request #10 from gerdr/master

Build system abstractions and Cygwin cross-compile support