Build OSX from source

[h-vetinari]

Fingers crossed that most of the infra from #60/#61 also works on osx.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

[h-vetinari]

Looks like upstream doesn't support building against (probably several) system deps on non-linux:

configure: error: Only bundled freetype can be specified on Mac and Windows

[h-vetinari]

@xhochy @isuruf
I have the first passing build here (so far only osx-64, something going wrong with the linker in cross-compilation, it thinks it's still on x64; f31312d was an attempted fix but is wrong) and wanted to ask for your feedback. For example, I'm not sure why the linux-side explicitly avoids dependence on iconv, I get missing symbols on osx if I do that.

The rest doesn't appear very complicated to me (the individual commits should be sensible to review one-by-one), just a bunch of platform differences.