FScript: fetch zipped sources, ib_plugin variant rewrite, xcode 4 check

[ci42]

changes:

• fetch the sources as a zip (upstream has added an appropriate tag on github)
• remove the without_ibplugin variant and add a (default) ib_plugin variant as a replacement (makes the portfile cleaner)
• add a pre-configure phase and check for xcode 4 as it does not support Imterface Builder plug-ins

[ryandesign (Ryan Carsten Schmidt)]

It would be nice if +ib_plugin were not a default variant when it doesn't work (i.e. when Xcode >= 4 is installed).

[ci42]

Replying to ryandesign@…:

It would be nice if +ib_plugin were not a default variant when it doesn't work (i.e. when Xcode >= 4 is installed).

Updated the patch so that the ib_plugin is only a default variant for Xcode < 4

[ci42]

added in r79814