Variables/PackageVariable.py only compares *default* against ENABLE_STRINGS

[dekeonus]

If SCons.Variables.PackageVariable.PackageVariable is initialised with a default "false" string, it will not be possible to enable the key by using a true boolean nor true string.

Given a SConscript file with something like

vars.Add(PackageVariable("icu", "ICU library location", "no"))

passing scons 'icu=yes' on the command line will not enable the key, only passing a /path will enable the key.
Moreover passing 'icu=yes' will result in a build failure:

scons: *** Path does not exist for variable 'icu': 'no'

This is because the PackageVariable _converter only compares default against ENABLE_STRINGS and so when a cli true value is passed the default value gets returned, however as a string.
The PackageVariable _validator will then parse this default value that should have been a bool as a string and will expect it to be a path.

scons/SCons/Variables/PackageVariable.py

Lines 87 to 89 in d54721e

	# Can't return the default if it is one of the enable/disable strings;
	# test code expects a bool.
	if default in ENABLE_STRINGS:

if this last line is modified to include DISABLE_STRINGS then the PackageVariable module works as expected. i.e.

    if default in ENABLE_STRINGS + DISABLE_STRINGS:

This is with scons-4.9.0 with python-3.12.9 on Gentoo amd64 (both installed by Gentoo's package manager)
Downstream Bug: https://bugs.gentoo.org/950584