Use a meson option to crossbuild for Windows

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>.

app/meson.build

@@ -19,7 +19,7 @@ src = [
     'src/tinyxpm.c',
 ]
 
-if not meson.is_cross_build()
+if not get_option('crossbuild_windows')
 
     # native build
     dependencies = [