remove some _MSC_VER stuff
Fix --disable-core-odbc-support

In commit ffc8e81b763c4e11e02309b99e2a4b4aa666e845, tc ensured that
configure would abort if libodbc was not found. However this resulted
in the library check being done twice, as well as rendering
--disable-core-odbc-support ineffective. If libodbc was found, it
would enable core ODBC support regardless. This fix ensures the check
is only done once or not at all if core ODBC support is explicitly
disabled.

Signed-off-by: Travis Cross <tc@traviscross.com>
Ensure we don't set HAVE_ODBC unless we really do
Improve configure ODBC handling