Give python users clear instructions about how to install cmake

setup.py

@@ -115,7 +115,45 @@ def get_cmake_version(self):
         try:
             out = subprocess.check_output(['cmake', '--version'])
         except:
-            sys.stderr.write("\nERROR: CMake must be installed to build dlib\n\n") 
+            sys.stderr.write("""
+================================================================================
+================================================================================
+================================================================================
+
+                   CMake is not installed on your system!
+
+    Or it is possible some broken copy of cmake is installed on your system.
+    It is unfortunately very common for python package managers to include
+    broken copies of cmake.  So if the error above this refers to some file
+    path to a cmake file inside a python or anaconda or miniconda path then you
+    should delete that broken copy of cmake from your computer.
+
+    Instead, please get an official copy of cmake from one of these known good
+    sources of an official cmake:
+        - cmake.org (this is how windows users should get cmake)
+        - apt install cmake (for Ubuntu or Debian based systems)
+        - yum install cmake (for Redhat or CenOS based systems)
+
+    On a linux machine you can run `which cmake` to see what cmake you are
+    actually using.  If it tells you it's some cmake from any kind of python
+    packager delete it and install an official cmake.
+
+    More generally, cmake is not installed if when you open a terminal window
+    and type 
+       cmake --version
+    you get an error.  So you can use that as a very basic test to see if you
+    have cmake installed.  That is, if cmake --version doesn't run from the
+    same terminal window from which you are reading this error message, then
+    you have not installed cmake.  Windows users should take note that they
+    need to tell the cmake installer to add cmake to their PATH.  Since you
+    can't run commands that are not in your PATH.  This is how the PATH works
+    on Linux as well, but failing to add cmake to the PATH is a particularly
+    common problem on windows and rarely a problem on Linux.
+
+================================================================================
+================================================================================
+================================================================================
+""") 
             sys.exit(1)
         return re.search(r'version\s*([\d.]+)', out.decode()).group(1)
 
@@ -199,8 +237,7 @@ def read_version_from_cmakelists(cmake_file):
     return major + '.' + minor + '.' + patch
 
 def read_entire_file(fname):
-    """Read text out of a file relative to setup.py.
-    """
+    """Read text out of a file relative to setup.py.  """
     return open(os.path.join(fname)).read()
 
 setup(