Code the unknown options check without using m4 diversions. Otherwise this

Code the unknown options check without using m4 diversions.  Otherwise this
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.