Bootstrap build problem after V8 test-in-container patch".

[DJ Delorie]

Sorry, been a busy couple of weeks.  Carlos and I did talk about this,
and the guidance he gave me was "If build-many-glibcs can build it, and
you can't, you need to do what build-many-glibcs does."

But otherwise, IMHO the build of the test program should be deferred
until we actually need it, but someone with more insight into how the
build rules works would need to help with that because I haven't figured
out how to get that work quite right yet :-P

Additionally, configure could be pickier about setting/clearing the CXX
variable, or failing if it's set to something unusable, but that's just
error protection rather than "make sure it works."