www/py-django40: unbreak build for HTML docs (+)

Django40 require at least sphinx 4.5.0 to generate offline HTML docs.
Set version check for dependency

Another issue is gmake-style construction in docs/Makefile which caused
build errors with non-default python.
Force gmake usage for HTMLDOCS option to resolve build issue.

Pointyhat to: wen (for skipped dependencies chech)
Approved by: portmgr blanket (fix build)