cmake: Revise external capnp search logic

No change in behavior, but the comments needed updating, so I updated them.
Then I noticed a never-true if() case and removed it.

c++/src/CMakeLists.txt

@@ -4,33 +4,31 @@
 if(BUILD_TESTING)
   include(CTest)
 
-  if (EXTERNAL_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()
+      # No working installation of Cap'n Proto found, so fall back to searching the environment.
+      #
+      # We search for the external capnp compiler binaries via $CAPNP, $CAPNPC_CXX, and
+      # find_program(). find_program() will use various paths in its search, among them
+      # ${CMAKE_PREFIX_PATH}/bin and $PATH.
+
+      if (NOT CAPNP_EXECUTABLE)
+        if (DEFINED ENV{CAPNP})
+          set(CAPNP_EXECUTABLE "$ENV{CAPNP}")
+        else()
+          find_program(CAPNP_EXECUTABLE capnp)
         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 "${capnp_dir}")
-          endif()
+      if(NOT CAPNPC_CXX_EXECUTABLE)
+        if (DEFINED ENV{CAPNPC_CXX})
+          set(CAPNPC_CXX_EXECUTABLE "$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}")
         endif()
       endif()
     endif()