script hack to set WITH_AFFINE variable with affine-extended removed

[gchabert]

I am going to remove the following lines in the plugins/wscript:

if (conf.options.WITH_AFFINE_EXTEND): conf.options.WITH_AFFINE =True

because if the affine-extended plugin is not installed, these lines make the script crash.

I have tried to replace the test by something like
if 'WITH_AFFINE_EXTEND' in conf.option.keys()
but it does not work neither.

I really think that this kind of hack is ugly anyway and that there should be only one affine plugin with an additional option to let the user chose a specific implementation, like, e.g.:

waf configure --with-affine --affine-implementation=...