Builder specific configs #13505

Is your feature request related to a problem? Please describe.
When building a standalone man page, I want to include a exclude_patterns so that the html files are not calculated. But I do not see a way to attach specific configs to a specific builder.

Describe the solution you'd like
Have a way to add builder specific options such as a builder_opts or expose a variable similar to tags to interogate which builder is being requested.

Describe alternatives you've considered
I have tried to include one dynamically via

def setup(app):
    if app.builder == "man":
        app.config.exclude_patterns = "**"

but this does not work because app.builder is not available at that point.