Accept prebuilt server

Expose a 'prebuilt_server' option to pass the path of the prebuilt
binary, so that the build does not require Android SDK.

Usage:

    meson builddir -Dprebuilt_server=/tmp/my_prebuilt_server.jar

meson_options.txt

@@ -1,3 +1,4 @@
 option('build_app', type: 'boolean', value: true)
 option('build_server', type: 'boolean', value: true)
+option('prebuilt_server', type: 'string')
 option('override_server_jar', type: 'string')

server/meson.build

@@ -1,7 +1,19 @@
-custom_target('scrcpy-server',
-              build_always: true,  # gradle is responsible for tracking source changes
-              input: '.',
-              output: 'scrcpy-server.jar',
-              command: [find_program('./scripts/build-wrapper.sh'), '@INPUT@', '@OUTPUT@', get_option('buildtype')],
-              install: true,
-              install_dir: 'share/scrcpy')
+# It may be useful to use a prebuilt server, so that no Android SDK is required
+# to build. If the 'prebuilt_server' option is set, just copy the file as is.
+prebuilt_server = get_option('prebuilt_server')
+if prebuilt_server == ''
+    custom_target('scrcpy-server',
+                  build_always: true,  # gradle is responsible for tracking source changes
+                  input: '.',
+                  output: 'scrcpy-server.jar',
+                  command: [find_program('./scripts/build-wrapper.sh'), '@INPUT@', '@OUTPUT@', get_option('buildtype')],
+                  install: true,
+                  install_dir: 'share/scrcpy')
+else
+    custom_target('scrcpy-server-prebuilt',
+                  input: prebuilt_server,
+                  output: 'scrcpy-server.jar',
+                  command: ['cp', '@INPUT@', '@OUTPUT@'],
+                  install: true,
+                  install_dir: 'share/scrcpy')
+endif