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 <Genymobile#165>.

Makefile.CrossWindows

@@ -55,6 +55,7 @@ build-win32: prepare-deps-win32
 		meson "$(WIN32_BUILD_DIR)" \
 			--cross-file cross_win32.txt \
 			--buildtype release --strip -Db_lto=true \
+			-Dcrossbuild_windows=true \
 			-Dbuild_server=false \
 			-Doverride_server_path=scrcpy-server.jar )
 	ninja -C "$(WIN32_BUILD_DIR)"
@@ -64,6 +65,7 @@ build-win32-noconsole: prepare-deps-win32
 		meson "$(WIN32_NOCONSOLE_BUILD_DIR)" \
 			--cross-file cross_win32.txt \
 			--buildtype release --strip -Db_lto=true \
+			-Dcrossbuild_windows=true \
 			-Dbuild_server=false \
 			-Dwindows_noconsole=true \
 			-Doverride_server_path=scrcpy-server.jar )
@@ -77,6 +79,7 @@ build-win64: prepare-deps-win64
 		meson "$(WIN64_BUILD_DIR)" \
 			--cross-file cross_win64.txt \
 			--buildtype release --strip -Db_lto=true \
+			-Dcrossbuild_windows=true \
 			-Dbuild_server=false \
 			-Doverride_server_path=scrcpy-server.jar )
 	ninja -C "$(WIN64_BUILD_DIR)"
@@ -86,6 +89,7 @@ build-win64-noconsole: prepare-deps-win64
 		meson "$(WIN64_NOCONSOLE_BUILD_DIR)" \
 			--cross-file cross_win64.txt \
 			--buildtype release --strip -Db_lto=true \
+			-Dcrossbuild_windows=true \
 			-Dbuild_server=false \
 			-Dwindows_noconsole=true \
 			-Doverride_server_path=scrcpy-server.jar )

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 = [

meson_options.txt

@@ -1,5 +1,6 @@
 option('build_app', type: 'boolean', value: true, description: 'Build the client')
 option('build_server', type: 'boolean', value: true, description: 'Build the server')
+option('crossbuild_windows', type: 'boolean', value: false, description: 'Build for Windows from Linux')
 option('windows_noconsole', type: 'boolean', value: false, description: 'Disable console on Windows (pass -mwindows flag)')
 option('prebuilt_server', type: 'string', description: 'Path of the prebuilt server')
 option('override_server_path', type: 'string', description: 'Hardcoded path to find the server at runtime')