Produce a clean failure in configure when dtrace is selected but cannot be

Produce a clean failure in configure when dtrace is selected but cannot be
found.  Besides stopping those early who have no dtrace installed
whatsoever, this will also alert those who have dtrace in /usr/sbin, which
might not be in the path, which would produce confusing failures much later
in the build process.

Add documentation about pointing configure to find dtrace.