Stop compiling protoc from source

[ninjudd]

Instead of compiling protoc from source, we should have users install protobuf using the package manager for their OS (brew, yum, etc...).

We may still have to download the Java source though, right @Raynes.

[ninjudd]

/assign @Raynes

[Raynes]

It looks like we extract some protos from the source distribution for our top-level protobuf project itself, so this isn't something we'll need to do for an end-user.

[ninjudd]

Great. So we can only download the source when building clojure-protobuf itself.

We should have a good error message for the user when protoc is not available though. Something like:

Could not find protoc on your path. You need to install it using your favorite package manager.
Try one of the following:
    brew install protobuf
    yum install protobuf
Or install it from source: http://code.google.com/p/protobuf/downloads/list

[ninjudd]

We changed our minds about this. Instead, we still compile protoc from source, we just don't install it using sudo.