configure: print warning for old compilers

Try to autodetect the C and C++ compiler and issue a warning when it's
too old to plausibly build io.js.  Emit warnings only because there is
much that can go wrong when trying to invoke a compiler.

PR-URL: https://github.com/iojs/io.js/pull/455


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>