cal: Use ALTMON_* correctly

cal: Use ALTMON_* correctly

cal: use ALTMON_* and _NL_ABALTMON_* constants to display
months in a standalone form correctly.  These constants have just
been newly added to glibc.  ALTMON_x has been used in BSD family
since 1990s and has been accepted as the future POSIX extension.
_NL_ABALTMON_* is exclusively a GNU extension but it is expected
to be added to POSIX in future.

More info: https://sourceware.org/bugzilla/show_bug.cgi?id=10871