MFC r250143, r257079, r258086, r262670:

r250143 (benno):
  Optimize SUBDIR_OVERRIDE such that SUBDIR isn't automatically
  defined if SUBDIR_OVERRIDE is defined.

r257079:
  Fix build host pollution by avoiding calling 'uname -srp' to
  determine values for 'VERSION'.

r262670 (marcel):
  Use ${MAKE} so that we always use the same version/implementation
  of make.

r258086 (cperciva):
  Strip the -pN patch level from the VERSION string which gets
  encoded into CTF data.  Otherwise FreeBSD Update builds think every
  kernel file has changed every time there's a security advisory,
  since the FreeBSD Update build code isn't smart enough to look inside
  CTF data to ignore those changes.

Tested on:	stable/9@r263881
Sponsored by:	The FreeBSD Foundation