ref-manual: Updates to CMake and cmake.bbclass.

I cleaned up the terminology used for the CMake build system.
I provided some meaningful links to a good overview page for CMake.
I also built in some cross-referencing as needed.

(From yocto-docs rev: 7e825427a2dcef2b91a1bfbf1827b73e034e0974)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-variables.xml

@@ -4633,7 +4633,12 @@
             <glossdef>
                 <para role="glossdeffirst">
 <!--                <para role="glossdeffirst"><imagedata fileref="figures/define-generic.png" /> -->
-                    Additional <filename>cmake</filename> options.
+                    Additional
+                    <ulink url='https://cmake.org/overview/'>CMake</ulink>
+                    options.
+                    See the
+                    <link linkend='ref-classes-cmake'><filename>cmake</filename></link>
+                    class for additional information.
                 </para>
             </glossdef>
         </glossentry>