Log Message:
Coalesce the common makefile code in the builtin.mk file that inspect
the X11 distribution using imake into mk/buildlink3/imake-check.mk.
imake-check.mk calls out to a helper shell script mk/buildlink3/imake-check
that generates the required Imakefiles and runs imake.  Remove the
now extraneous builtin-imake.mk files as the builtin.mk files can now
contain the name of the imake symbol to check.