Small fix to import statement.

Explicitly import from . - it's a relative import.
VS: Allow config-dependent values in custom compile commands.

All of the values involved (output file names, commands, dependencies)
may be configuration or arch dependent, so they have to be evaluated
through the config proxy object.
Better handling of conditionally set required properties.

Bakefile would fail on pretty much any property set conditionally if it
was required, even if it was used by only some toolsets and set only for
them, or if the tests covered all possibilities.

Be smarted about this, defer the test until later, into
make_variables_for_missing_props(), and don't throw prematurely.
Warn about use of backslash as path separator.