"mbdtools.h" couldn't be included from C++ files, because the "glib.h"
include is in the extern "C" block. The latter includes C++ headers
for C++ source files, which define templates. These don't work with
extern "C" linkage and the compilation falls apart with numerous
errors.
To fix this problem, move the includes out of the extern "C" block.
Use wcstombs and friends to perform Unicode conversion inside the ODBC
driver. Functionality should be identical as before, except that
installations without iconv will now be able to compile and install the
Unicode (libmdbodbcW) driver.
iconv remains present in the main libmdb library.
Add new mdbi_register_backend2 function with the features introduced in
PRs #321 and #322. The purpose is to prevent API/ABI breakage in the
1.0 release.
