Don't warn about ASCII encodings (subset of UTF-8)

Currently warnings are thrown by concatenate-source-op every time ASDF
is loaded if UTF-8 is the default encoding because the ASDF source is
ASCII which is not equal to UTF-8.  This commit adds a special case to
the encoding equality check so that ASCII is recognized as a subset of
UTF-8.

Author: eschulte

concatenate-source.lisp

@@ -69,7 +69,8 @@ into a single file"))
           :append
           (when (typep c 'cl-source-file)
             (let ((e (component-encoding c)))
-              (unless (equal e encoding)
+              (unless (or (equal e encoding)
+                          (and (equal e :ASCII) (equal encoding :UTF-8)))
                 (let ((a (assoc e other-encodings)))
                   (if a (push (component-find-path c) (cdr a))
                       (push (list e (component-find-path c)) other-encodings)))))