Change subdir processing so that SUBDIRS will contain the configured

Change subdir processing so that SUBDIRS will contain the configured
subdirs only for the makefile in the directory with the configure
script, and will have only $(LOCAL_SUBDIRS) elsewhere.  Drop the use
of "MY_SUBDIRS=." to override SUBDIRS in favor of this way of keeping
SUBDIRS empty.  Drop other uses of MY_SUBDIRS in favor of
LOCAL_SUBDIRS or (in one case, the top level) overriding the SUBDIRS
setting from pre.in.

One less thing to keep tweaking as configure scripts get reorganized
and merged.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@16158 dc483132-0cff-0310-8789-dd5450dbe970