Support DBD ODBC driver as an optional feature, defaulting to ON
since the libraries are always available on Windows.
(A fair amount of this is from Tom Donovan's cmake list.)

Enable the DBD test driver when the test suite is being built.

Add/clarify a small amount of commentary about the lack of
a test suite build for libapr-2 (the DLL) and the manner in
which DBD driver are expected to be built.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1516863 13f79535-47bb-0310-9956-ffa450edef68