This upgrades the configure infrastructure to the latest Autoconf version.
Some notable news are:
- The workaround for the broken fseeko() test is gone.
- Checking for unknown options is now provided by Autoconf itself.
- Fixes for Mac OS X
code relies on the checking macro actually being called at the end, or the
automatic undiversion will produce garbage. These sort of implicit
side-effects undermine the modularity of the macros and happen to break the
ODBC driver which makes use of them.
Also put the warnings at the very end of configure, so there is an even
better chance of seeing them.