Improved error messages for people who have incorrectly installed boost in windows.

dlib/cmake_utils/add_python_module

@@ -100,33 +100,44 @@ endif()
 if (NOT Boost_FOUND)
     message(STATUS " *****************************************************************************************************")
     if (WIN32)
-        message(STATUS "We couldn't find the right version of boost python.  If you installed boost and you are still "
-                "getting this error then you might have installed a version of boost that was compiled with a different "
-                "version of visual studio than the one you are using.  So you have to make sure that the version of "
-                "visual studio is the same version that was used to compile the copy of boost you are using.")
-        message(STATUS " Set the BOOST_ROOT and BOOST_LIBRARYDIR environment variables before running cmake. ")
-        message(STATUS " E.g.  Something like this: ")
-        message(STATUS "    set BOOST_ROOT=C:\\local\\boost_1_57_0 " )
-        message(STATUS "    set BOOST_LIBRARYDIR=C:\\local\\boost_1_57_0\\stage\\lib")
-        message(STATUS "")
-        message(STATUS " You will also likely need to compile boost yourself rather than using one of the precompiled ")
-        message(STATUS " windows binaries.  Do this by going to the folder tools\\build\\ within boost and running ")
-        message(STATUS " bootstrap.bat.  Then run the command: ")
-        message(STATUS "    b2 install")
-        message(STATUS " And then add the output bin folder to your PATH.  Usually this is the C:\\boost-build-engine\\bin")
-        message(STATUS " folder. Finally, go to the boost root and run a command like this:")
-        message(STATUS "    b2 -a --with-python address-model=64 toolset=msvc runtime-link=static")
-        message(STATUS " When it completes, set BOOST_LIBRARYDIR equal to wherever b2 put the compiled libraries.")
-        message(STATUS " Note that you will need to set the address-model based on if you want a 32 or 64bit python library.")
-        message(STATUS "")
-        message(STATUS " Next, if you aren't using python setup.py then you will be invoking cmake to compile dlib.  ")
-        message(STATUS " In this case you may have to use cmake's -G option to set the 64 vs. 32bit mode of visual studio. ")
-        message(STATUS " Also, if you want a Python3 library you will need to add -DPYTHON3=1.  You do this with a statement like: ")
-        message(STATUS "    cmake -G \"Visual Studio 14 2015 Win64\" -DPYTHON3=1 ..\\..\\tools\\python")
-        message(STATUS " Rather than:")
-        message(STATUS "    cmake ..\\..\\tools\\python")
-        message(STATUS " Which will build a 32bit Python2 module by default on most systems.")
-        message(STATUS "")
+        if (NOT DEFINED ENV{BOOST_ROOT} OR NOT DEFINED ENV{BOOST_LIBRARYDIR})
+            message(STATUS "The BOOST_ROOT and/or BOOST_LIBRARYDIR environment variables are not set.  You need to set these ")
+            message(STATUS "variables so we know where boost is installed.  So set these variables to something like this:")
+            message(STATUS "    BOOST_ROOT=C:\\local\\boost_1_57_0 " )
+            message(STATUS "    BOOST_LIBRARYDIR=C:\\local\\boost_1_57_0\\stage\\lib")
+            message(STATUS "The best way to set enviornment variables in Windows is to go to the start menu's search box,")
+            message(STATUS "type environment, and open the Environment Variables dialog.  That lets you add more variables.")
+            message(STATUS "Add these variables to either user or system variables, it doesn't matter.")
+            message(STATUS "You might have to log out and log back in before the environment changes take effect.")
+        else()
+            message(STATUS "We couldn't find the right version of boost python.  If you installed boost and you are still "
+                    "getting this error then you might have installed a version of boost that was compiled with a different "
+                    "version of visual studio than the one you are using.  So you have to make sure that the version of "
+                    "visual studio is the same version that was used to compile the copy of boost you are using.")
+            message(STATUS "")
+            message(STATUS " You will likely need to compile boost yourself rather than using one of the precompiled ")
+            message(STATUS " windows binaries.  Do this by going to the folder tools\\build\\ within boost and running ")
+            message(STATUS " bootstrap.bat.  Then run the command: ")
+            message(STATUS "    b2 install")
+            message(STATUS " And then add the output bin folder to your PATH.  Usually this is the C:\\boost-build-engine\\bin")
+            message(STATUS " folder. Finally, go to the boost root and run a command like this:")
+            message(STATUS "    b2 -a --with-python address-model=64 toolset=msvc runtime-link=static")
+            message(STATUS " Note that you will need to set the address-model based on if you want a 32 or 64bit python library.")
+            message(STATUS " When it completes, set the BOOST_LIBRARYDIR environment variable equal to wherever b2 put the ")
+            message(STATUS " compiled libraries.  You will also need to set BOOST_ROOT to the root folder of the boost install.")
+            message(STATUS " E.g.  Something like this: ")
+            message(STATUS "    set BOOST_ROOT=C:\\local\\boost_1_57_0 " )
+            message(STATUS "    set BOOST_LIBRARYDIR=C:\\local\\boost_1_57_0\\stage\\lib")
+            message(STATUS "")
+            message(STATUS " Next, if you aren't using python setup.py then you will be invoking cmake to compile dlib.  ")
+            message(STATUS " In this case you may have to use cmake's -G option to set the 64 vs. 32bit mode of visual studio. ")
+            message(STATUS " Also, if you want a Python3 library you will need to add -DPYTHON3=1.  You do this with a statement like: ")
+            message(STATUS "    cmake -G \"Visual Studio 14 2015 Win64\" -DPYTHON3=1 ..\\..\\tools\\python")
+            message(STATUS " Rather than:")
+            message(STATUS "    cmake ..\\..\\tools\\python")
+            message(STATUS " Which will build a 32bit Python2 module by default on most systems.")
+            message(STATUS "")
+        endif()
     else()
         message(STATUS " To compile Boost.Python yourself download boost from boost.org and then go into the boost root folder")
         message(STATUS " and run these commands: ")