Don't use make while building Prism

[tenderlove]

This commit teaches mkmf about the libprism sources and allows it to compile them directly

Fixes #3273

I was able to teach mkmf about the libprism source files without copying them in to the ext folder. The code isn't particularly nice, you have to know about global variables internal to mkmf, but it does eliminate calling make during the gem build process.

@mame do you think this will work on mswin64? I don't have a machine handy to test with.

[mame]

I confirmed that a custom-built prism gem with your patch worked well on my windows. Thank you!

C:\Users\YusukeEndoh>.\ruby\local\bin\gem install prism-1.2.0.gem
Building native extensions. This could take a while...
Successfully installed prism-1.2.0
Ignoring prism-0.27.0 because its extensions are not built. Try: gem pristine prism --version 0.27.0
Parsing documentation for prism-1.2.0
Installing ri documentation for prism-1.2.0
Done installing documentation for prism after 37 seconds
1 gem installed