Firebird: fix harmless warning in 64 bit builds.

In 64 bit builds Firebird defines isc_stmt_handle as an integer type and not a
pointer, so comparing it with or assigning NULL to it results in gcc warnings.

Avoid them by using 0 instead of NULL, this works in both 32 bit (where
isc_stmt_handle is a pointer) and 64 bit builds.
Fix windows.h case for cross-compilation

Patch from Michael Surette received through soci-users.

While the Windows filesystems are case insensitive, other systems are case sensitive.  While cross-compiling all lower case filenames seem to be the norm.  Both MinGW and Cygwin seem to have windows.h in their APIs.  See http://sourceforge.net/apps/trac/cppcheck/ticket/4636 for a short discussion on this issue.