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

@@ -6,37 +6,40 @@ include_directories("${CMAKE_CURRENT_SOURCE_DIR}")
 if(BUILD_TESTING)
   include(CTest)
 
-  # Setup CAPNP_GENERATE_CPP for compiling test schemas
-  find_package(CapnProto CONFIG QUIET)
-  if (NOT CapnProto_FOUND)
-    # Try and find the executables from an autotools-based installation
-    # Setup paths to the schema compiler for generating ${test_capnp_files}
-    if(NOT EXTERNAL_CAPNP AND NOT CAPNP_LITE)
-      set(CAPNP_EXECUTABLE $<TARGET_FILE:capnp_tool>)
-      set(CAPNPC_CXX_EXECUTABLE $<TARGET_FILE:capnpc_cpp>)
-    else()
-      # Allow paths to tools to be set with either environment variables or find_program()
-      if (NOT CAPNP_EXECUTABLE)
-        if (DEFINED ENV{CAPNP})
-          set(CAPNP_EXECUTABLE "$ENV{CAPNP}")
-        else()
-          find_program(CAPNP_EXECUTABLE capnp)
+  if (EXTERNAL_CAPNP)
+    # Setup CAPNP_GENERATE_CPP for compiling test schemas
+    find_package(CapnProto CONFIG QUIET)
+    if (NOT CapnProto_FOUND)
+      # Try and find the executables from an autotools-based installation
+      # Setup paths to the schema compiler for generating ${test_capnp_files}
+      if(NOT EXTERNAL_CAPNP AND NOT CAPNP_LITE)
+        set(CAPNP_EXECUTABLE $<TARGET_FILE:capnp_tool>)
+        set(CAPNPC_CXX_EXECUTABLE $<TARGET_FILE:capnpc_cpp>)
+      else()
+        # Allow paths to tools to be set with either environment variables or find_program()
+        if (NOT CAPNP_EXECUTABLE)
+            if (DEFINED ENV{CAPNP})
+            set(CAPNP_EXECUTABLE "$ENV{CAPNP}")
+            else()
+            find_program(CAPNP_EXECUTABLE capnp)
+            endif()
         endif()
-      endif()
 
-      if(NOT CAPNPC_CXX_EXECUTABLE)
-        if (DEFINED ENV{CAPNPC_CXX})
-          set(CAPNPC_CXX_EXECUTABLE "$ENV{CAPNPC_CXX}")
-        else()
-          # Also search in the same directory that `capnp` was found in
-          get_filename_component(capnp_dir "${CAPNP_EXECUTABLE}" DIRECTORY)
-          find_program(CAPNPC_CXX_EXECUTABLE capnpc-c++ HINTS "${capnp_dir}")
+        if(NOT CAPNPC_CXX_EXECUTABLE)
+          if (DEFINED ENV{CAPNPC_CXX})
+            set(CAPNPC_CXX_EXECUTABLE "$ENV{CAPNPC_CXX}")
+          else()
+            # Also search in the same directory that `capnp` was found in
+            get_filename_component(capnp_dir "${CAPNP_EXECUTABLE}" DIRECTORY)
+            find_program(CAPNPC_CXX_EXECUTABLE capnpc-c++ HINTS "${capnp_dir}")
+          endif()
         endif()
       endif()
     endif()
-  endif()
 
-  set(CAPNP_INCLUDE_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")
+    set(CAPNP_INCLUDE_DIRECTORY "${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)