* lib/canonicalize.h: (canonicalize_mode_t, CAN_EXISTING):

* lib/canonicalize.h: (canonicalize_mode_t, CAN_EXISTING):
(CAN_ALL_BUT_LAST, CAN_MISSING, canonicalize_filename_mode):
Define only if PROVIDE_CANONICALIZE_FILENAME_MODE is defined.
* m4/canonicalize.m4 (AC_FUNC_CANONICALIZE_FILE_NAME):
Check for functions and headers just once.
Check for declaration of canonicalize_file_name.
Define PROVIDE_CANONICALIZE_FILENAME_MODE.