Compile libgraphqlparser when compiling graphql_parser

[anthonator]

I think it would be handy to compile libgraphqlparser as part graphql_parser's compile process.

I'm assuming you're not already doing this because of this line in the README:

But I'd recommend building and installing from source, because the library is constantly updated with critical bug fixes. It's in pretty early stages, so this is the most recommended approach.

It doesn't look like libgraphqlparser is "constantly updated" (last release was October 2017; last commit was October 2018) or "in pretty early stages" anymore (although it is pre-1.0).

I'm not exactly sure how to do this but I think elixir_make exists to make this possible.

I'd be happy to look into implementing this.