Convert targeted sdk flycheck option from list to string

swiftc frontend doesn't support multiple parameters for the `--sdk` key
Use correct function from cl-lib
Correct source argument extraction in flycheck checker
Shortcut linked sources flycheck logic when corresponding variable is nil
Don't active flycheck checker by default

It doesn't work with linked libraries, this means it doesn't work well
with Xcode projects.
Update README
Merge pull request #56 from ap4y/fix_flycheck_issues

Fix flycheck issues