Replace meson subprojects by subdir

Since Meson 0.44, subproject_dir may not be '.' anymore. This implies we
must move app/ and server/ to a subprojects/ directory, which requires
to also change some gradle files.

Instead, just use subdir(), with options to disable building of the app
or the server.

Makefile

@@ -34,7 +34,7 @@ build-portable:
 	[ -d "$(PORTABLE_BUILD_DIR)" ] || ( mkdir "$(PORTABLE_BUILD_DIR)" && \
 		meson "$(PORTABLE_BUILD_DIR)" \
 			--buildtype release --strip -Db_lto=true \
-			-Dapp:override_server_jar=scrcpy-server.jar )
+			-Doverride_server_jar=scrcpy-server.jar )
 	ninja -C "$(PORTABLE_BUILD_DIR)"
 
 release-portable: clean dist-portable-zip sums

app/meson.build

@@ -1,5 +1,3 @@
-project('scrcpy-app', 'c')
-
 src = [
     'src/main.c',
     'src/command.c',

app/meson_options.txt

@@ -1 +0,0 @@
-option('override_server_jar', type: 'string')

meson.build

@@ -1,6 +1,11 @@
-project('scrcpy', 'c', subproject_dir: '.')
+project('scrcpy', 'c')
 
-subproject('app')
-subproject('server')
+if get_option('build_app')
+    subdir('app')
+endif
+
+if get_option('build_server')
+    subdir('server')
+endif
 
 run_target('run', command: ['scripts/run-scrcpy.sh'])

meson_options.txt

@@ -0,0 +1,3 @@
+option('build_app', type: 'boolean', value: true)
+option('build_server', type: 'boolean', value: true)
+option('override_server_jar', type: 'string')