Keep pyexpander generated files out of build directory

The previous behavior of the pyexpander check (in
VTKmCheckPyexpander.cmake) was to generate the file in the binary
directory and then remove it from there iff the check failed. This
caused two problems. The first is that if the check failed then the file
was deleted and there was no file to copy to the source as the
instructions suggested. The second is that if the check succeeded the
build would then use the files in the build directory rather than the
source directory, and if the programmer accidently modified the binary
files (because, for example, if a build error occured there), the
configure system would not catch that.

This change in behavior was that if the check failed, the file is
renamed to have a .save extension so that the file remains and can be
easily copied back to the source directory if that is appropriate. If
the check succeeds, the generated file is removed and a file with the
extension .check is touched. That .check file is used as a make target
to signal that the test has been performed.

CMake/VTKmCheckPyexpander.cmake

@@ -63,9 +63,22 @@ execute_process(
 
 if(${diff_result})
   # If diff returned non-zero, it failed and the two files are different.
-  file(REMOVE ${GENERATED_FILE})
   get_filename_component(filename ${SOURCE_FILE} NAME)
+  # Move the generated file so that the build does not confuse it with the
+  # files in the source directory.
+  file(REMOVE ${GENERATED_FILE}.save)
+  file(RENAME ${GENERATED_FILE} ${GENERATED_FILE}.save)
   message(SEND_ERROR
     "The source file ${filename} does not match the generated file. If you have modified this file directly, then you have messed up. Modify the ${filename}.in file instead and then copy the pyexpander result to ${filename}. If you modified ${filename}.in, then you might just need to copy the pyresult back to the source directory. If you have not modifed either, then you have likely checked out an inappropriate change. Check the git logs to see what changes were made.
-If the changes have resulted from modifying ${filename}.in, then you can finish by copying ${GENERATED_FILE} to ${SOURCE_FILE}.")
+If the changes have resulted from modifying ${filename}.in, then you can finish by moving ${GENERATED_FILE}.save over ${SOURCE_FILE}")
+else()
+  # Now that we have done the comparison, remove the generated file so there is
+  # no confusion between the generated files and the source files checked into
+  # the repository.
+  file(REMOVE ${GENERATED_FILE})
+  # Pyexpander successfully checked, so touch a file to tell make when the
+  # check was last successfully performed.
+  execute_process(
+    COMMAND ${CMAKE_COMMAND} -E touch ${GENERATED_FILE}.checked
+    )
 endif()

CMake/VTKmMacros.cmake

@@ -135,7 +135,7 @@ function(vtkm_pyexpander_generated_file generated_file_name)
   # If pyexpander is available, add targets to build and check
   if(PYEXPANDER_FOUND)
     add_custom_command(
-      OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${generated_file_name}
+      OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${generated_file_name}.checked
       COMMAND ${CMAKE_COMMAND}
         -DPYEXPANDER_COMMAND=${PYEXPANDER_COMMAND}
         -DSOURCE_FILE=${CMAKE_CURRENT_SOURCE_DIR}/${generated_file_name}
@@ -143,10 +143,10 @@ function(vtkm_pyexpander_generated_file generated_file_name)
         -P ${CMAKE_SOURCE_DIR}/CMake/VTKmCheckPyexpander.cmake
       MAIN_DEPENDENCY ${CMAKE_CURRENT_SOURCE_DIR}/${generated_file_name}.in
       DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/${generated_file_name}
+      COMMENT "Checking validity of ${generated_file_name}"
       )
     add_custom_target(check_${generated_file_name} ALL
-      DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${generated_file_name}
-      COMMENT "Checking validity of ${generated_file_name}"
+      DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${generated_file_name}.checked
       )
   endif()
 endfunction(vtkm_pyexpander_generated_file)