scripts: kernel-doc: allow passing desired Sphinx C domain dialect

When kernel-doc is called via kerneldoc.py, there's no need to
auto-detect the Sphinx version, as the Sphinx module already
knows it. So, add an optional parameter to allow changing the
Sphinx dialect.

As kernel-doc can also be manually called, keep the auto-detection
logic if the parameter was not specified. On such case, emit
a warning if sphinx-build can't be found at PATH.

I ended using a suggestion from Joe for using a more readable
regex, instead of using a complex one with a hidden group like:

	m/^(\d+)\.(\d+)(?:\.?(\d+)?)/

in order to get the optional <patch> argument.

Thanks-to: Joe Perches <joe@perches.com>
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/sphinx/kerneldoc.py

@@ -70,6 +70,11 @@ class KernelDocDirective(Directive):
         env = self.state.document.settings.env
         cmd = [env.config.kerneldoc_bin, '-rst', '-enable-lineno']
 
+	# Pass the version string to kernel-doc, as it needs to use a different
+	# dialect, depending what the C domain supports for each specific
+	# Sphinx versions
+        cmd += ['-sphinx-version', sphinx.__version__]
+
         filename = env.config.kerneldoc_srctree + '/' + self.arguments[0]
         export_file_patterns = []