Fix 'FindCapnProto.cmake' for CMake versions <2.8.12.
CMake: Basic check for C++11, and setup global install paths.
CMake: Add support for `CAPNP_LITE` option (see [c772a70]).

With CAPNP_LITE=1, the command-line tools will not be built, and the
tests cannot be built without EXTERNAL_CAPNP=1 (BUILD_TESTING=0 needs to
be set if the tests are not desired).
CMake: Generate pkgconfig files for installation.
CMake: Add support for detecting the 'lite' library in 'FindCapnProto.cmake'.

Also, use pkg-config (if available) to get hints about the paths.
CMake: Consistently quote variables that may contain special characters. Fixes #136.
CMake: Replace URL_HASH with URL_MD5 for CMake versions <2.8.11.2