Make a --with-freetype option instead of having to uncomment

[lehoff]

In premake4.lua there was a comment about what to uncomment when needing FreeType support.
This has now been turned into a premake option that can be passed in as a command line option like this:

$ premake4 --with-freetype gmake

[cdegroot]

By the way, the reason for me for having to use Freetype is that when I tried NanoVG with the STB Truetype library on a RaspberryPi using VideoCore, strange things happened. The errors that came back indicated a failure to allocate memory, but looking at malloc traces that is unlikely, so it seems that in this particular configuration, the STB code writes in a wrong place. Freetype works fine (and I guess should render somewhat nicer, even if it enlarges the footprint of NanoVG a bit?). Currently I just forked NanoVG with a patched premake4.lue but having a command line option would be nicer.

So consider this my: yes, please, accept this PR ;-)