Added messages saying how to install Boost on UNIX systems if it isn't

installed already.

dlib/add_python_module

@@ -15,6 +15,11 @@
 
 option(PYTHON3 "Build a Python3 compatible library rather than Python2." OFF)
 
+# Avoid cmake warnings about changes in behavior of some Mac OS X path 
+# variable we don't care about.
+if (POLICY CMP0042)
+    cmake_policy(SET CMP0042 NEW)
+endif()
 
 #SET(Boost_USE_STATIC_LIBS OFF)
 #SET(Boost_USE_MULTITHREADED ON)
@@ -37,29 +42,41 @@ else()
     FIND_PACKAGE(Boost 1.41.0 COMPONENTS python REQUIRED)
     FIND_PACKAGE(PythonLibs 2.6 REQUIRED)
 endif()
-if (NOT Boost_FOUND AND WIN32)
-    message(" Set the BOOST_ROOT and BOOST_LIBRARYDIR environment variables before running cmake. ")
-    message(" E.g.  Something like this: ")
-    message("    set BOOST_ROOT=C:\\local\\boost_1_57_0 " )
-    message("    set BOOST_LIBRARYDIR=C:\\local\\boost_1_57_0\\stage\\lib")
-    message("")
-    message(" You will also likely need to compile boost yourself rather than using one of the precompiled ")
-    message(" windows binaries.  Do this by going to the folder tools\\build\\ within boost and running ")
-    message(" bootstrap.bat.  Then run the command: ")
-    message("    b2 install")
-    message(" And then add the output bin folder to your PATH.  Usually this is the C:\\boost-build-engine\\bin")
-    message(" folder. Finally, go to the boost root and run a command like this:")
-    message("    b2 --with-python address-model=64 toolset=msvc --build-type=complete")
-    message(" And set the BOOST_LIBRARYDIR equal to wherever it puts the compiled libraries.")
-    message(" Note that you will need to set the address-model based on if you want a 32 or 64bit python library.")
-    message(" Similarly, when you invoke cmake you may have to use cmake's -G option to set the 64 vs. 32bit mode")
-    message(" of visual studio.  Also, if you want a Python3 library will also need to add -DPYTHON3=1.  You do ")
-    message(" this with a statement like: ")
-    message("    cmake -G \"Visual Studio 12 2013 Win64\" -DPYTHON3=1 ..\\..\\tools\\python")
-    message(" Rather than:")
-    message("    cmake ..\\..\\tools\\python")
-    message(" Which will build a 32bit Python2 module by default on most systems.")
-    message("")
+if (NOT Boost_FOUND)
+    if (WIN32)
+        message(" Set the BOOST_ROOT and BOOST_LIBRARYDIR environment variables before running cmake. ")
+        message(" E.g.  Something like this: ")
+        message("    set BOOST_ROOT=C:\\local\\boost_1_57_0 " )
+        message("    set BOOST_LIBRARYDIR=C:\\local\\boost_1_57_0\\stage\\lib")
+        message("")
+        message(" You will also likely need to compile boost yourself rather than using one of the precompiled ")
+        message(" windows binaries.  Do this by going to the folder tools\\build\\ within boost and running ")
+        message(" bootstrap.bat.  Then run the command: ")
+        message("    b2 install")
+        message(" And then add the output bin folder to your PATH.  Usually this is the C:\\boost-build-engine\\bin")
+        message(" folder. Finally, go to the boost root and run a command like this:")
+        message("    b2 --with-python address-model=64 toolset=msvc --build-type=complete")
+        message(" And set the BOOST_LIBRARYDIR equal to wherever it puts the compiled libraries.")
+        message(" Note that you will need to set the address-model based on if you want a 32 or 64bit python library.")
+        message(" Similarly, when you invoke cmake you may have to use cmake's -G option to set the 64 vs. 32bit mode")
+        message(" of visual studio.  Also, if you want a Python3 library will also need to add -DPYTHON3=1.  You do ")
+        message(" this with a statement like: ")
+        message("    cmake -G \"Visual Studio 12 2013 Win64\" -DPYTHON3=1 ..\\..\\tools\\python")
+        message(" Rather than:")
+        message("    cmake ..\\..\\tools\\python")
+        message(" Which will build a 32bit Python2 module by default on most systems.")
+        message("")
+    else()
+        message("")
+        message(" *****************************************************************************************************")
+        message(" To compile Boost.Python yourself download boost from boost.org and then go into the boost root folder")
+        message(" and run these commands: ")
+        message("    ./bootstrap.sh --with-libraries=python") 
+        message("    ./b2")
+        message("    sudo ./b2 install")
+        message(" *****************************************************************************************************")
+        message("")
+    endif()
 endif()
 
 if (WIN32 AND NOT Boost_LIBRARIES)