[kroplabeskidu]

Description

Items 52 and 53 from #6159.

• message(AUTHOR_WARNING, ...): the comma makes the first argument AUTHOR_WARNING,, which isn't a message mode, so the text was printed as a plain message starting with AUTHOR_WARNING,. Fixed at both call sites.
• The .venv autodetection checked ENV{VIRTUALENV}, but the variable is VIRTUAL_ENV, so a .venv in the source tree was used even with another venv active. With VIRTUAL_ENV set, configuring no longer prints Autodetecting Python in virtual environment.

I checked both by hand with CMake. There are no new tests.

Suggested changelog entry:

• Fixed the SETUPTOOLS_EXT_SUFFIX mismatch warning so it is issued as an author warning, and made .venv autodetection respect an active VIRTUAL_ENV.