In r257079, SRCDIR is pointed to ${.CURDIR} when not set.  However,
Makefile.inc1 is being called in sub-make's where make(1) would,
by default, implicitly chdir(2) to ${.OBJDIR} before executing any
targets.  This would make some targets, like delete-old, when trying
to derive various variables introduced by change r256921 using
``make -f Makefile.inc1'' that also rely on SRCDIR to fail.

This changeset adds an explicit cd ${.CURDIR} before these unwrapped
make calls, making them in line with the other ones that are already
being wrapped with the explicit chdir's.