Simplify portable build configuration

To create a portable build (with scrcpy-server.jar accessible from the
scrcpy directory), replace OVERRIDE_SERVER_PATH by a simple compilation
flag: PORTABLE.

This paves the way to use more complex rules to determine the path of
scrcpy-server.jar in portable builds.

app/src/server.c

@@ -12,8 +12,8 @@
 
 #define SOCKET_NAME "scrcpy"
 
-#ifdef OVERRIDE_SERVER_PATH
-# define DEFAULT_SERVER_PATH OVERRIDE_SERVER_PATH
+#ifdef PORTABLE
+# define DEFAULT_SERVER_PATH "scrcpy-server.jar"
 #else
 # define DEFAULT_SERVER_PATH PREFIX "/share/scrcpy/scrcpy-server.jar"
 #endif