fixits: handle %file-prefix

* src/files.h, src/files.c (spec_file_prefix_loc): New.
* src/scan-gram.l (%file-prefix): Delegate diagnostics to...
* src/parse-gram.y (handle_file_prefix): here.
* src/complain.c (duplicate_directive): Quote the directive.
* tests/input.at: Adjust.

tests/local.at

@@ -707,8 +707,9 @@ if test "$POSIXLY_CORRECT_IS_EXPORTED" = false; then
   # Build expected stderr up to and including the "warnings being
   # treated as errors" message.
   ]AT_DATA([[experr]], [$4])[
-  $PERL -pi -e 's{(.*): warning: (.*)\[-W(.*)\]$}
-                 {$][1: error: $][2\@<:@-Werror=$][3@:>@}' experr
+  $PERL -pi -e 's{(.*): warning:}{$][1: error:};' \
+            -e 's{\[-W(.*)\]$}{@<:@-Werror=$][1@:>@}' \
+                 experr
   ]AT_CHECK([[sed 's,.*/$,,' stderr 1>&2]], [[0]], [[]], [experr])[
 
   # Now check --warnings=error.