Bugfix/code clean using gcc warnings (#371)

* Enable extra GCC warnings to discover subtle bugs

* convert c++ comments to c comments

* cleanup pedantic compiler warnings

* Compile apps with GNU89 GNU99 GNU11 and GNU17

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

apps/mstpcap/main.c

@@ -401,11 +401,13 @@ static void packet_statistics_clear(void)
 
 static uint32_t Timer_Silence(void *pArg)
 {
+    (void)pArg;
     return mstimer_elapsed(&Silence_Timer);
 }
 
 static void Timer_Silence_Reset(void *pArg)
 {
+    (void)pArg;
     mstimer_set(&Silence_Timer, 0);
 }
 
@@ -924,9 +926,9 @@ static void print_help(char *filename)
     printf("Captures MS/TP packets from a serial interface\n"
            "and saves them to a file. Saves packets in a\n"
            "filename mstp_20090123091200.cap that has data and time.\n"
-           "After receiving 65535 packets, a new file is created.\n"
-           "\n"
-           "Command line options:\n"
+           "After receiving 65535 packets, a new file is created.\n");
+    printf("\n");
+    printf("Command line options:\n"
            "[--extcap-interface port] - serial interface.\n"
 #if defined(_WIN32)
            "    Supported values: COM1, COM2, etc.\n"