libntp: clean up MKREPRO_TIMESTAMP handling

NetBSD's make has built-in support for formatting timestamps, so use
that instead of relying on an external tool.  The month name is still
always in the C locale, and possible errors are reported in the affected
line, due to the ':=' assignment operator.

Without the ':=' assignment operator, the intermediate variable would
not be necessary, but in that case, make's error handling is broken and
unspecific.

(rillig)