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 +104,23 @@ cmd_simple_wait(pid_t pid, int *exit_code) {
     }
     return !code;
 }
+
+char *
+get_executable_path(void) {
+// <https://stackoverflow.com/a/1024937/1987178>
+#ifdef __linux__
+    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);
+#else
+    // in practice, we only need this feature for portable builds, only used on
+    // Windows, so we don't care implementing it for every platform
+    // (it's useful to have a working version on Linux for debugging though)
+    return NULL;
+#endif
+}