Improve portable builds

In portable builds, scrcpy-server.jar was supposed to be present in the
current directory, so in practice it worked only if scrcpy was launched
from its own directory.

Instead, find the absolute path of the executable and build a suitable
path to use scrcpy-server.jar from the same directory.

app/src/sys/unix/command.c

@@ -1,9 +1,15 @@
+// for portability
 #define _POSIX_SOURCE // for kill()
+#define _BSD_SOURCE // for readlink()
+
+// modern glibc will complain without this
+#define _DEFAULT_SOURCE
 
 #include "command.h"
 
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <signal.h>
 #include <stdlib.h>
 #include <sys/types.h>
@@ -98,3 +104,15 @@ cmd_simple_wait(pid_t pid, int *exit_code) {
     }
     return !code;
 }
+
+char *
+get_executable_path(void) {
+    char buf[PATH_MAX + 1]; // +1 for the null byte
+    ssize_t len = readlink("/proc/self/exe", buf, PATH_MAX);
+    if (len == -1) {
+        perror("readlink");
+        return NULL;
+    }
+    buf[len] = '\0';
+    return SDL_strdup(buf);
+}