build: Purge docs before rebuilding.

If a help file is renamed, stale help files in the build workspace will
cause duplicate tags (which causes the build to fail). To avoid this,
always delete build/runtime/doc/ before building helptags.

runtime/CMakeLists.txt

@@ -26,23 +26,15 @@ foreach(DF ${DOCFILES})
   list(APPEND BUILDDOCFILES ${GENERATED_RUNTIME_DIR}/doc/${F})
 endforeach()
 
-add_custom_command(OUTPUT copy_docfiles
-    COMMAND ${CMAKE_COMMAND} -E copy_directory
-    ${PROJECT_SOURCE_DIR}/runtime/doc ${GENERATED_RUNTIME_DIR}/doc
-)
-
 add_custom_target(helptags
+  COMMAND ${CMAKE_COMMAND} -E remove_directory ${GENERATED_RUNTIME_DIR}/doc
+  COMMAND ${CMAKE_COMMAND} -E copy_directory
+    ${PROJECT_SOURCE_DIR}/runtime/doc ${GENERATED_RUNTIME_DIR}/doc
   COMMAND "${PROJECT_BINARY_DIR}/bin/nvim"
-    -u NONE
-    -i NONE
-    -e
-    --headless
-    -c "helptags ++t ."
-    -c quit
+    -u NONE -i NONE -e --headless -c "helptags ++t doc" -c quit
   DEPENDS
-    copy_docfiles
     nvim
-  WORKING_DIRECTORY "${GENERATED_RUNTIME_DIR}/doc"
+  WORKING_DIRECTORY "${GENERATED_RUNTIME_DIR}"
 )
 
 add_custom_command(OUTPUT ${GENERATED_HELP_TAGS}