Fix feature test macro
The expected feature test macro is _POSIX_C_SOURCE having a value
greater or equal to 200809L.
Fixes <https://github.com/Genymobile/scrcpy/issues/1726>
-// 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
