Pass --config-settings of the form 'option=value' as SCons ARGUMENTS

as suggested in https://github.com/dholth/enscons/issues/36

Example:

in SConstruct, add

for k in ARGUMENTS:
    print(k, "->", ARGUMENTS[k])

if building a wheel with pip wheel python-sample/ --config-settings 'a=b' --verbose --verbose --verbose you'll see lines a -> b in the log.