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)
@@ -37,7 +42,8 @@ else()
     FIND_PACKAGE(Boost 1.41.0 COMPONENTS python REQUIRED)
     FIND_PACKAGE(PythonLibs 2.6 REQUIRED)
 endif()
-if (NOT Boost_FOUND AND WIN32)
+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 " )
@@ -60,6 +66,17 @@ if (NOT Boost_FOUND AND WIN32)
         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)