Add support for WITHOUT_MOSQUITTO

I'm doing builds for my jail (which doesn't have libmosquitto) on
my system that does have it now, so I need to disable it manually.

src/sbbs3/GNUmakefile

@@ -89,6 +89,7 @@ ifeq ($(os),linux)
   CON_LIBS += -lsystemd
  endif
 endif
+ifndef WITHOUT_MOSQUITTO
  ifneq ($(os), win32)
   ifeq ($(shell pkg-config libmosquitto --exists && echo "yes"),yes)
    CFLAGS += -DUSE_MOSQUITTO
@@ -97,6 +98,7 @@ ifneq ($(os), win32)
    JS_LIBS += $(shell pkg-config libmosquitto --libs)
   endif
  endif
+endif
 
 include sbbsdefs.mk
 MT_CFLAGS	+=	$(SBBSDEFS)