Leave CAPNP{,C_CXX}_EXECUTABLE unset if tools aren't built

When building Cap'n Proto with compilers which can't yet build the command
line tools (MSVC), the exported targets capnp_tool and capnpc_cpp don't
exist, thus attempting to use them in a generator expression when setting
CAPNP_EXECUTABLE and CAPNPC_CXX_EXECUTABLE causes a hard error.

Leaving the variables unset provides users the opportunity to set
CAPNP_EXECUTABLE and CAPNPC_CXX_EXECUTABLE manually in CMake projects
consuming Cap'n Proto as a CMake package, and produces a less mystifying
error message if the user forgets.

Thanks to Philip Quinn for advice on this issue.

c++/cmake/CapnProtoConfig.cmake.in

@@ -12,8 +12,10 @@
 
 set(CapnProto_VERSION @VERSION@)
 
-set(CAPNP_EXECUTABLE $<TARGET_FILE:CapnProto::capnp_tool>)
-set(CAPNPC_CXX_EXECUTABLE $<TARGET_FILE:CapnProto::capnpc_cpp>)
+if(@CAPNP_BUILD_TOOLS@)
+  set(CAPNP_EXECUTABLE $<TARGET_FILE:CapnProto::capnp_tool>)
+  set(CAPNPC_CXX_EXECUTABLE $<TARGET_FILE:CapnProto::capnpc_cpp>)
+endif()
 set(CAPNP_INCLUDE_DIRECTORY "@PACKAGE_CMAKE_INSTALL_FULL_INCLUDEDIR@")
 
 # work around http://public.kitware.com/Bug/view.php?id=15258