c-capnproto: add package

c-capnproto: add package

Adds the c-capnproto package. This creates a C plugin for the regular
capnproto capnpc compiler. It supports only serialization (no RPC).

Since it depends on capnproto, the package selects capnproto and has the
same requirements as capnproto.

The host version is always needed for the code generator, which is what
gets called by packages using it. The target version is also always
needed for the library. Therefore, no Config.in.host option is needed.

We need to use the git download method because the package has
submodules. The submodule is gtest, which is not really needed, but it's
complicated to remove the dependency.

The version is the latest upstream git hash. It is version 0.3 with just
two patches applied: one to the README file, and a fix submitted by us.

Signed-off-by: Joel Carlson <JoelsonCarl@gmail.com>
[Arnout:
 - use latest upstream version which has patch already applied;
 - mark in Config.in that dependencies come from capnproto;
 - add some newlines to .mk file;
 - updated commit message.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>