build: avoid compiler warnings

build: avoid compiler warnings

* lib/fchmodat.c (lchmod): Mark unused variables.
* lib/getopt.c (_getopt_initialize): Likewise.
* lib/mktime.c (__mktime_internal): Provide prototype.
* lib/inttostr.c (inttostr): Avoid compiler warning even with
older gcc that do not understand #pragma GCC diagnostic.
* lib/uinttostr.c (inttype_is_unsigned): Define.
* lib/umaxtostr.c (inttype_is_unsigned): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>