gcc.c (struct prefix_list): Add member priority.

* gcc.c (struct prefix_list): Add member priority.
(enum path_prefix_priority): Declare.
(add_prefix): Replace ``first'' with ``priority''.  Append new
entry but keep list in priority order.
(process_command): Update.  Pass PREFIX_PRIORITY_B_OPT or
PREFIX_PRIORITY_LAST to add_prefix.
(process_command): Move include kludge - foo/stageN - to before
foo/include.

From-SVN: r35304