Force use of Swift 3 compatibility mode to compile sources.

Author: tkremenek

lib/phases.py

@@ -172,6 +172,8 @@ def generate(self):
         generated += " -I" + Configuration.current.build_directory.path_by_appending(self.product.name).relative() + self.product.ROOT_HEADERS_FOLDER_PATH
         generated += " -I" + Configuration.current.build_directory.relative()
         swiftflags = TargetConditional.value(self.product.SWIFTCFLAGS)
+        # Force building in Swift 3 compatibility mode.
+        swiftflags += " -swift-version 3"
         if swiftflags is not None:
             generated += " " + swiftflags
         return generated