build: define helptags target which always runs.

Specify that the ${GENERATED_HELP_TAGS} "command" (output) depends on
`helptags` so that it always regenerates the doc/ tags. (cmake "targets"
always run, whereas "commands" are contingent on their dependencies. But
we don't define doc/ dependencies because they are circular.)

Makefile

@@ -85,10 +85,10 @@ endif
 	mkdir -p build
 	touch $@
 
-oldtest: | nvim tags
+oldtest: | nvim helptags
 	+$(SINGLE_MAKE) -C src/nvim/testdir $(MAKEOVERRIDES)
 
-tags: | nvim
+helptags: | nvim
 	+$(BUILD_CMD) -C build runtime/doc/tags
 
 functionaltest: | nvim

runtime/CMakeLists.txt

@@ -31,7 +31,7 @@ add_custom_command(OUTPUT copy_docfiles
     ${PROJECT_SOURCE_DIR}/runtime/doc ${GENERATED_RUNTIME_DIR}/doc
 )
 
-add_custom_command(OUTPUT ${GENERATED_HELP_TAGS}
+add_custom_target(helptags
   COMMAND "${PROJECT_BINARY_DIR}/bin/nvim"
     -u NONE
     -i NONE
@@ -45,6 +45,11 @@ add_custom_command(OUTPUT ${GENERATED_HELP_TAGS}
   WORKING_DIRECTORY "${GENERATED_RUNTIME_DIR}/doc"
 )
 
+add_custom_command(OUTPUT ${GENERATED_HELP_TAGS}
+  DEPENDS
+    helptags
+)
+
 add_custom_target(doc_html
   COMMAND make html
   DEPENDS