cmake: do not require EXTERNAL_CAPNP for CAPNP_LITE without BUILD_TESTING

c++/CMakeLists.txt

@@ -30,8 +30,8 @@ option(EXTERNAL_CAPNP "Use the system capnp binary, or the one specified in $CAP
 option(CAPNP_LITE "Compile Cap'n Proto in 'lite mode', in which all reflection APIs (schema.h, dynamic.h, etc.) are not included. Produces a smaller library at the cost of features. All programs built against the library must be compiled with -DCAPNP_LITE. Requires EXTERNAL_CAPNP." OFF)
 
 # Check for invalid combinations of build options
-if(CAPNP_LITE AND NOT EXTERNAL_CAPNP)
-  message(SEND_ERROR "You must set EXTERNAL_CAPNP when using CAPNP_LITE.")
+if(CAPNP_LITE AND BUILD_TESTING AND NOT EXTERNAL_CAPNP)
+  message(SEND_ERROR "You must set EXTERNAL_CAPNP when using CAPNP_LITE and BUILD_TESTING.")
 endif()