Pull up following revision(s) (requested by jakllsch in ticket #663):

external/bsd/ntp/lib/libntp/Makefile: revision 1.30
external/bsd/ntp/lib/libntp/Makefile: revision 1.31
external/bsd/ntp/lib/libntp/Makefile: revision 1.28
external/bsd/ntp/lib/libntp/Makefile: revision 1.29

use ${MKREPRO_TIMESTAMP} for baking in the date and time.
fix format.
remove now-unused assignment
Format MKREPRO_TIMESTAMP with "%b %d %Y" to correctly substitute __DATE__

(martin)