Avoid compiling xcassets in some cases

Added a condition to avoid compiling xcassets when WRAPPER_EXTENSION
is undefined, as it would be in the case of static libraries. This prevents
trying to copy the compiled files to a directory that does not exist.

See https://github.com/CocoaPods/CocoaPods/pull/1427#issuecomment-27025766 
for additional details. Essentially, `xcassets` were always being compiled regardless of if
they were needed or not. 

Quick question: I notice that some scripts use `[[` as opposed to `[`, should we use one over the other?