cmake: speed up libcurl doc building again

cmake: speed up libcurl doc building again

This time limit the number of files per command to avoid exceeding
limitations of certain OS/shell envs.

Such known env is Windows with the `cmd.exe` shell, which features an
8K command-line length limit to this day.

Allowlisting `UNIX` to have no limit and using a limit of 200 for other
envs to be safe. If there is a way to detect `cmd.exe` and/or we know
which precise envs are sensitive to this, we can tweak these conditions
further.

Even with the low limit, this patch reduces external commands by 200x,
making builds much faster.

Ref: #12762 2620aa930bc73af1e4c70b10e3125b957b96ecfb (initial)
Ref: #13047 f03c85635f35269f1f45b983bf216624f541760a (revert)

Reviewed-by: Daniel Stenberg
Closes #13207