Add more details on how to build Mosquitto.

Explains more of the differences between the Makefiles and the CMake
scripts.

CMakeLists.txt

@@ -1,4 +1,8 @@
-# This is a cmake script.
+# This is a cmake script. Process it with the CMake gui or command line utility
+# to produce makefiles / Visual Studio project files on Mac OS X and Windows.
+#
+# To configure the build options either use the CMake gui, or run the command
+# line utility including the "-i" option.
 
 set(CMAKE_LEGACY_CYGWIN_WIN32 0)
 

Makefile

@@ -14,6 +14,10 @@ docs :
 binary : mosquitto
 
 mosquitto :
+ifeq ($(UNAME),Darwin)
+ $(error Please compile using CMake on Mac OS X)
+endif
+
  set -e; for d in ${DIRS}; do $(MAKE) -C $${d}; done
 
 clean :

config.mk

@@ -1,6 +1,9 @@
 # =============================================================================
 # User configuration section.
 #
+# These options control compilation on all systems apart from Windows and Mac
+# OS X. Use CMake to compile on Windows and Mac.
+#
 # Largely, these are options that are designed to make mosquitto run more
 # easily in restrictive environments by removing features.
 #
@@ -94,6 +97,7 @@ DB_HTML_XSL=man/html.xsl
 #MANCOUNTRIES=en_GB
 
 UNAME:=$(shell uname -s)
+
 ifeq ($(UNAME),SunOS)
  ifeq ($(CC),cc)
  CFLAGS?=-O