Fix feature test macro

The expected feature test macro is _POSIX_C_SOURCE having a value
greater or equal to 200809L.

Fixes #1726 <https://github.com/Genymobile/scrcpy/issues/1726>

app/src/sys/unix/command.c

-// for portability
-#define _POSIX_SOURCE // for kill()
-#define _BSD_SOURCE // for readlink()
+// for portability (kill, readlink, strdup, strtok_r)
+#define _POSIX_C_SOURCE 200809L
+#define _BSD_SOURCE
 
 // modern glibc will complain without this
 #define _DEFAULT_SOURCE