Add option USE_SUBMODULE_GTEST

The option is designed for projects that already use gtest and do want
to reuse it in capnp.

Gtest could be incorporated in CMake-based project in different ways.
Some distributions provide binary packages for gtest, and in such cases
the most common approach is to use find_package(GTest).

On the other hand, it is recommended to build gtest among with the
project that uses it to ensure that exactly the same complitation flags
and environment variables are used. For those who want to follow this
guideline there are two possible options: add gtest as external project
using ExternalProject_Add, or incorporate gtest into current build tree using
add-subdirectory(path/to/submodule/gtest). Option USE_SUBMODULE_GTEST is
mostly useful for the latter case.

Sample usage:

git submodule add git://some.repository.with.gtest/gtest externals/gtest
git submodule add https://github.com/isn-/capnproto.git externals/capnproto

CMakeLists.txt:

add_subdirectory(externals/gtest)
set(USE_SUBMODULE_GTEST ON CACHE BOOL "Use gtest from git submodule")
add_subdirectory(externals/capnproto)

add_executable(my-capnp-test main.cpp)
target_link_libraries(my-capnp-test capnp gtest gtest_main)