cmake: don't use find_package(CapnProto) if EXTERNAL_CAPNP is not set

Using installed capnp tools during instead of built capnp tools
easily leads to bad results.

c++/src/CMakeLists.txt

@@ -6,6 +6,7 @@ include_directories("${CMAKE_CURRENT_SOURCE_DIR}")
 if(BUILD_TESTING)
   include(CTest)
 
+  if (EXTERNAL_CAPNP)
     # Setup CAPNP_GENERATE_CPP for compiling test schemas
     find_package(CapnProto CONFIG QUIET)
     if (NOT CapnProto_FOUND)
@@ -37,6 +38,8 @@ if(BUILD_TESTING)
     endif()
 
     set(CAPNP_INCLUDE_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")
+    #TODO(someday) It would be nice to use targets instead of variables in CAPNP_GENERATE_CPP macro
+  endif()
 
   # Sadly, we can't use the 'test' target, as that's coopted by ctest
   add_custom_target(check "${CMAKE_CTEST_COMMAND}" -V)