Support disabling builds of the command-line tools and unit tests, and using an external compiler.

c++/CMakeLists.txt

 
+option(BUILD_TOOLS "Build command-line tools and compiler." ON)
+option(BUILD_TESTING "Build unit tests and enable CTest 'check' target." ON)
+option(EXTERNAL_CAPNP "Use the system capnp binary, or the one specified in $CAPNP, instead of using the compiled one." OFF)
+
+if (NOT BUILD_TOOLS AND BUILD_TESTING AND NOT EXTERNAL_CAPNP)
+  message(WARNING "Forcing BUILD_TOOLS to ON; required by BUILD_TESTING without EXTERNAL_CAPNP.")
+  set(BUILD_TOOLS TRUE)
+endif()
+
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wno-unused-parameter -std=c++11 -pthread")
 
 add_subdirectory(src)