MFC r327439:

Allow use of .WAIT in the LOCAL_DIRS and LOCAL_LIB_DIRS lists.

A comment in Makefile.inc1 has long stated that LOCAL_DIRS are built last,
after the base system. Incremental improvements in parallel building over
the years have led to LOCAL_DIRS being built in parallel with base system
directories. This change allows the .WAIT directive to appear in LOCAL_DIRS
and LOCAL_LIB_DIRS lists to give the user some control over parallel
building of local additions.

Differential Revision: https://reviews.freebsd.org/D13622