[SCons] Add option to skip running Sphinx Gallery examples

SConstruct

@@ -308,6 +308,12 @@ config_options = [
         "sphinx_docs",
         "Build HTML documentation for Cantera using Sphinx.",
         False),
+    BoolOption(
+        "run_examples",
+        """Run examples to generate plots and outputs for Sphinx Gallery. Disable to
+           speed up doc builds when not working on the examples or if dependencies of
+           the examples are not available.""",
+        True),
     PathOption(
         "sphinx_cmd",
         "Command to use for building the Sphinx documentation.",

doc/SConscript

@@ -34,6 +34,8 @@ if localenv['sphinx_docs']:
     localenv.PrependENVPath('PYTHONPATH', Dir('#build/python').abspath)
     def build_sphinx(target, source, env):
         cmd = [env['sphinx_cmd']] + env['sphinx_options'].split()
+        if not env['run_examples']:
+            cmd.extend(('-D', 'plot_gallery=0'))
         if env['logging'] == 'debug':
             cmd.append('-v')
         cmd += ('-b', 'html', '-d', 'build/doc/sphinx/doctrees', 'build/doc/sphinx',