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.