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

@@ -89,12 +89,14 @@ ifeq ($(os),linux)
   CON_LIBS += -lsystemd
  endif
 endif
-ifneq ($(os), win32)
- ifeq ($(shell pkg-config libmosquitto --exists && echo "yes"),yes)
-  CFLAGS += -DUSE_MOSQUITTO
-  CFLAGS += $(shell pkg-config libmosquitto --cflags)
-  CON_LIBS += $(shell pkg-config libmosquitto --libs)
-  JS_LIBS += $(shell pkg-config libmosquitto --libs)
+ifndef WITHOUT_MOSQUITTO
+ ifneq ($(os), win32)
+  ifeq ($(shell pkg-config libmosquitto --exists && echo "yes"),yes)
+   CFLAGS += -DUSE_MOSQUITTO
+   CFLAGS += $(shell pkg-config libmosquitto --cflags)
+   CON_LIBS += $(shell pkg-config libmosquitto --libs)
+   JS_LIBS += $(shell pkg-config libmosquitto --libs)
+  endif
  endif
 endif