Small update to gold plugin docs for building with CMake.

[srking]

Summary

Describe how to set the include path to binutils when building with CMake.

[srking]

Use fixed point font for include path.

[t.p.northover]

Good idea. Do you have commit access or would you like me to do it?

Tim.

[Eugene.Zelenko]

Committed in rL221227.

docs/GoldPlugin.rst

.. code-block:: bash
      $ cd build
      $ ../binutils/configure --enable-gold --enable-plugins --disable-werror
      $ make all-gold
 
   That should leave you with ``build/gold/ld-new`` which supports
   the ``-plugin`` option. Running ``make`` will additionally build
   ``build/binutils/ar`` and ``nm-new`` binaries supporting plugins.
 
-* Build the LLVMgold plugin: Configure LLVM with
-  ``--with-binutils-include=/path/to/binutils/include`` and run
-  ``make``.
+* Build the LLVMgold plugin.  If building with autotools, run configure with
+  ``--with-binutils-include=/path/to/binutils/include`` and run ``make``.
+  If building with CMake, run cmake with
+  ``-DLLVM_BINUTILS_INCDIR=/path/to/binutils/include``.  The correct include
+  path will contain the file ``plugin-api.h``.
 
 Usage
 =====
 
 The linker takes a ``-plugin`` option that points to the path of
 the plugin ``.so`` file. To find out what link command ``gcc``
 would run in a given situation, run ``gcc -v [...]`` and
 look for the line where it runs ``collect2``. Replace that with