main: fix handling of -g option if feature is disabled

Without this patch building the agent with:

  make CFLAGS=-DENABLE_GdbRemoteSerialProtocol=0

And running it with:

  ./agent -g2000

would silently swollow the option and continue with the default
behaviour, leaving the user with the impression of a running
gdb-rsp server - which is not the case. In addition the option
-g should not show up in the usage string.

This patch fixes both, remove the option from the usage and bail out
with an error if -g is specified but ENABLE_GdbRemoteSerial is zero.

Change-Id: I9a1f0396e7e8c2c2844cc89bdde8ff03cbbf337d
Signed-off-by: Sanimir Agovic <sanimir@subpath.org>
diff --git a/agent/tcf/main/main.c b/agent/tcf/main/main.c
index 51c84e8..7d765c9 100644
--- a/agent/tcf/main/main.c
+++ b/agent/tcf/main/main.c
@@ -220,7 +220,7 @@
 #endif
     "  -s<url>          set agent listening port and protocol, default is " DEFAULT_SERVER_URL,
     "  -S               print server properties in Json format to stdout",
-#if ENABLE_DebugContext
+#if ENABLE_GdbRemoteSerialProtocol
     "  -g<port>         start GDB Remote Serial Protocol server at the specified TCP port",
 #endif
     "  -I<idle-seconds> exit if there are no connections for the specified time",
@@ -346,7 +346,9 @@
 #endif
             case 'L':
             case 's':
+#if ENABLE_GdbRemoteSerialProtocol
             case 'g':
+#endif
 #if ENABLE_Plugins
             case 'P':
 #endif