Deal with bmake-20170301 no longer resolving -C like it used to.

Several checks assume .CURDIR is resolved, such as for determining RELDIR from
SRCTOP/.CURDIR. If -C is used then the path is no longer resolved like it was
before which is problematic for symlinked source trees. A similar change was
also made to ports post bmake-20170301.

This fixes 'make -C <symlinked path> buildworld' using the wrong OBJDIR.

Reported by: rstone
Sponsored by: Dell EMC