Switch to a supported build system

[jvoisin]

scalloc is using gyp, which isn't supported anymore, and doesn't work on current versions of Python.
Would it be possible to switch to a supported build system?

We'd like to keep scalloc in mimalloc bench (see daanx/mimalloc-bench#96 ).

[ckirsch]

@mlippautz Hi Michael, any chance you can look into this?

[ajccosta]

Hopefully PR #3 fixes these issues.