meson: add 'sphinx-build' to sphinx executable name list
if get_option('documentation')
-cmd_sphinx = find_program('sphinx-build-3', 'sphinx-build3')
+cmd_sphinx = find_program('sphinx-build-3', 'sphinx-build3', 'sphinx-build')
doc_config = configuration_data()
doc_config.set('PACKAGE_NAME', meson.project_name())
