Make that returned character "signed" explicity so that it is always a signed character on all platforms.

shairport-sync-dbus-test-client.c

@@ -73,7 +73,7 @@ int main(int argc, char *argv[]) {
 
   GBusType gbus_type_selected = G_BUS_TYPE_SYSTEM; // set default
   // get the options --system or --session for system bus or session bus
-   char    c;            /* used for argument parsing */
+   signed char c;            /* used for argument parsing */
    poptContext optCon;   /* context for parsing command-line options */
 
    struct poptOption optionsTable[] = {

shairport-sync-mpris-test-client.c

@@ -51,7 +51,7 @@ void *dbus_thread_func(void *arg) {
 int main(int argc, char *argv[]) {
   GBusType gbus_type_selected = G_BUS_TYPE_SYSTEM; // set default
   // get the options --system or --session for system bus or session bus
-   char    c;            /* used for argument parsing */
+   signed char c;            /* used for argument parsing */
    poptContext optCon;   /* context for parsing command-line options */
 
    struct poptOption optionsTable[] = {