Backed out rev.1.136 and rev.1.144. Force SHARED=symlinks in a different

Backed out rev.1.136 and rev.1.144.  Force SHARED=symlinks in a different
way - for buildworld only.

Rev.1.136 broke the (deprecated) external includes target.  Rev.1.144 was
a half-baked backout of rev.1.136.  It broke SHARED=symlinks for the
buildworld target and installation of subdirs of src/include for the
includes target.