Xcode project: set architecture to i386

This is required for the included libs (pango, cairo, etc.).  The
libraries should be updated at some point, but until that happens this
is what needs to be used.
Xcode project: set gcc version to Xcode default for wesnothd

This was already done and accidentally committed together with
d9659f69b4a7d5b2d8a2b1ef9acab08d291c885f for Wesnoth
Xcode project: enable strict compilation