update package libusb1

The 1.0.19 release now uses VerSetConditionMask(), which is not
available by default in i686-pc-mingw32.static (but it *is* for
i686-w64-mingw32.static).

Thus, add 'CFLAGS=-D_WIN32_WINNT=0x0500' like in other packages
to make libusb1 also compile for i686-pc-mingw32.static.