Don't strip binaries when building through Xcode

Without this change building the Release version results in an error (which
claims to be a warning) due to using -s which is obsolete.
This removes -s when building through xcode - gnumake and other platforms are
(presumably) unaffected.
Merge pull request #38 from goetzk/change-linker-invocation-through-xcode

Don't strip binaries when building through Xcode