Move perform method of cc-flags-mixin around process-op
Move perform method of cc-flags-mixin around process-op

+ The c files are generated during the process-op operation, not the
  compile-op operation. This fixes a bug where specifiying :cc-flags
  in the asdf file had no effect.

dkochmanski/cffi,stassats/cffi,cffi/cffi,stassats/cffi,3b/cffi,3b/cffi,dkochmanski/cffi,3b/cffi,dkochmanski/cffi,cffi/cffi,stassats/cffi,cffi/cffi