Meson decided to crossbuild for Windows as soon as
meson.is_cross_build() returned true. This made non-Windows crossbuilds
fail.
Instead, add an explicit option "crossbuild_windows".
Fixes <https://github.com/Genymobile/scrcpy/issues/165>.
On Windows, an application is either console or gui, it cannot be both.
Scrcpy should be both: it outputs important information to console, but
we still want to be able to ignore the console and launch it without a
visible cmd.exe window.
Therefore, build two binaries:
- scrcpy.exe
- scrcpy-noconsole.exe
Build the Windows binary from mingw on Linux, using the official
prebuilt binaries for ffmpeg, SDL2 and adb.
MSYS2 and all its packaged dll are not necessary anymore.
