Warn about being pnacl-specific

[kornelski]

The library seems to work only with pnacl. To use it with system library I've had to make it regenerate the bindings on the fly: kornelski@562e479

I'm not sure if you want to adopt the change, because it depends on bindgen, which depends on the whole llvm and clang.

[DiamondLovesYou]

I wonder if the story is better than it used to be. The specific checkouts required to match Rust's LLVM with a version of Clang that could be compiled meant that using bindgen was a PITA. This was years ago, so I'll bet that it has gotten better.

Honestly, due to the lack of community around PNaCl (especially now that webassembly is being developed) made me stop developing for it due to the headaches one gets when doing everything yourself.
Long live Wasm.

If you want to issue a PR for that changeset, I'll merge it.

[kornelski]

So the current situation is:

• libvpx requires declared abi version to match, so to use shared system library, the ffi has to be generated from the system headers having a matching version.
• generating ffi on the fly using bindgen is problematic: it requires clang and llvm, which are big dependencies, and "stable" linux distros have old versions which are unsupported or buggy

I'm wating for bindgen to release the latest round of changes that fixed problems with libvpx headers.

bindgen is a heavy dependency, so I'm contemplating shipping multiple versions of pre-generated ffi.rs instead.

I know nothing about PNaCl or Wasm generation though. My use cases are purely for native applications.

[DiamondLovesYou]

Using a separate ffi.rs for each platform is reasonable alternative for the time being.