CMake improvements

A few improvements to make it easier to integrate Cap'n Proto with other CMake projects.

Highlights:
* Use `ExternalProject` to pull and build googletest out-of-source as part of the `make`.
* Rudimentary install support (mainly to help other projects to find the outputs and public headers, rather than install it on the system).
* `FindCapnProto.cmake` module to locate libraries and compile schema files.
* Options to disable tool/test targets, or use an external compiler when cross-compiling.

Shared libraries aren't produced because I couldn't get the output names to match what the `automake` build does.