PackageVariable now returns the default on "true"

[mwichmann]

In all doc versions until 4.8.0, PackageVariable had wording like: The option will support the values yes, true, on, enable or search, in which case the specified default will be used, but the code didn't actually do that, it just returned True. With this change it now returns the default value, with a slight tweak - if the default is one of the spelled out enabling strings, it returns the boolean True instead. This should be harmless - the produced value is still going to evaluate true, even if it can now be a path string.

The indication that the default is produced if a truthy string is given is restored to the manpage (it was never dropped from the User Guide).

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation

[mwichmann]

Windows fail on the test addition, because the tested path ended up not properly escaped. Will investigate.

STDOUT =========================================================================
scons: Reading SConscript files ...

STDERR =========================================================================
  File "C:\Users\mats\AppData\Local\Temp\scons\testcmd.30516.xh_86zgt\SConstruct.path", line 8

    default='C:\Users\mats\AppData\Local\Temp\scons\testcmd.30516.xh_86zgt\path\to\tinycbor'

            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 2-3: truncated \UXXXXXXXX escape

[mwichmann]

Windows fail on the test addition, because the tested path ended up not properly escaped. Will investigate.

Forgot the r to make it a rawstring - one-character fix. Works for me on Windows now.