Fix creating build directory for unknown build target

[jbking]

Subject: Fix creating build directory for unknown build target

Feature or Bugfix

• Bugfix

Purpose

• Fix building target specified to make command becomes build directory name even unknown target or typo.
  Sphinx ensures their existence at bootstrap. So unexpected build directory was created.

Relates

• Sphinx build make mode creates empty directories for unknown targets/builders #4093

[tk0miya]

Note: The directory creation has been moved after these processing:

• execute conf.py (but it could not refers outdir)
• setup_extension() for bundled and user-given extensions
• preload_builder (call setup_extension() for given builder-name)

As a result, app.outdir might point a non-existing directory during setup extensions.
This is surely incompatible change, but IMO it does not harm anybody.
So +1 for this!

[shimizukawa]

LGTM!

[tk0miya]

Thank you for your contribution!