MAINT: Check for doxygen first.

Makefile

@@ -80,6 +80,7 @@ conan: conan-install-deps conan-create
 .PHONY: conan
 
 doc:
+	@doxygen --version || ( echo "Doxygen not found.  Please install Doxygen first." ; exit 1 )
 	mkdir -p build && cd build && conan install .. --build=missing -s $(CONAN_SETTINGS) \
 	  && cmake .. && cmake --build . --target doc-sphinx \
 	  && echo "The documentation starts at build/doc/sphinx_out/index.html ."