SDK: allow finding CLT SDK path where present

Library/Homebrew/os/mac/sdk.rb

@@ -82,7 +82,7 @@ module OS
       # return nil SDKs for Xcode 9 and older.
       def sdk_prefix
         @sdk_prefix ||= begin
-          if !CLT.separate_header_package?
+          if !CLT.provides_sdk?
             ""
           else
             "#{CLT::PKG_PATH}/SDKs"

Library/Homebrew/os/mac/xcode.rb

@@ -214,6 +214,10 @@ module OS
         version >= "10"
       end
 
+      def provides_sdk?
+        version >= "8"
+      end
+
       def headers_installed?
         if !separate_header_package?
           installed?