gtkdochelper.py: Use os.pathsep for --path argument

This way, we can ensure that gtk-doc parses the --path argument
correctly when passed in from the cmd.exe console, since ':' is normally
used to denote that a drive is being used on local paths.

mesonbuild/scripts/gtkdochelper.py

@@ -198,7 +198,7 @@ def build_gtkdoc(source_root: str, build_root: str, doc_subdir: str, src_subdirs
 
     # Make HTML documentation
     mkhtml_cmd = [options.gtkdoc_mkhtml,
-                  '--path=' + ':'.join((doc_src, abs_out)),
+                  '--path=' + os.pathsep.join((doc_src, abs_out)),
                   module,
                   ] + html_args
     if main_file: