graphite2/icu api support

[ratmice]

I have a really terrible set of patches, for building harfbuzz with graphite2/icu support,
and setting environment variables via CARGO_DEP_* so that further C libraries can link against everything,

graphite2 stuff is based upon a lightly modified version of @spl's graphite2-sys crate.

I guess I am curious what the appropriate route to upstreaming is (after getting rid of some of the terribleness),
I assume that I should implement cargo features for icu and graphite2, and control the --enable lines from features?

I haven't really used any sys libs with optional features before, so i'm curious if this is amicable to upstream...

master...ratmice:master

[waywardmonkeys]

This would conflict with the changes that I'm also working on in #170 ... Have you seen those? It might simplify some of the build issues on the Harfbuzz side.

[ratmice]

I've briefly looked at them, and believe I understand the gist of it,

I'm guessing that, I'd need to make some cargo features, and in the big amalgamation source file, these features would need #ifdef or whatever macro conditional, and then have cc crate define these on compile? Anyhow I will have a look at implementing it atop that patch and see how it goes.