DOC: skip generating c/c++ doc from comment blocks when Doxygen or Br…

…eathe isn't available.

doc/Makefile

@@ -178,7 +178,11 @@ html: version-check html-build
 html-build: generate
 	mkdir -p build/html build/doctrees
 	$(PYTHON) preprocess.py
+ifeq (, $(shell which $(DOXYGEN)))
+	@echo "Unable to find 'Doxygen:$(DOXYGEN)', skip generating C/C++ API from comment blocks."
+else
 	$(DOXYGEN) build/doxygen/Doxyfile
+endif
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) build/html $(FILES)
 	$(PYTHON) postprocess.py html build/html/*.html
 	@echo

doc/source/conf.py

@@ -2,6 +2,7 @@
 import os
 import re
 import sys
+import importlib
 
 # Minimum version, enforced by sphinx
 needs_sphinx = '3.2.0'
@@ -84,9 +85,18 @@ class PyTypeObject(ctypes.Structure):
     'IPython.sphinxext.ipython_console_highlighting',
     'IPython.sphinxext.ipython_directive',
     'sphinx.ext.mathjax',
-    'breathe'
 ]
 
+skippable_extensions = [
+    ('breathe', 'skip generating C/C++ API from comment blocks.'),
+]
+for ext, warn in skippable_extensions:
+    ext_exist = importlib.util.find_spec(ext) is not None
+    if ext_exist:
+        extensions.append(ext)
+    else:
+        print(f"Unable to find Sphinx extension '{ext}', {warn}.")
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']