Blender Git Loki

CMake: expand instructions for accidental 'cmake' run in source dir

Our CMake setup refuses to run from the source directory (i.e. Blender does
not support in-source builds). Instead, it shows instructions on how to
clean up after an accidental `cmake` invocation. These instructions missed
one directory that should also be removed (`CMakeFiles`), so that's been
added to the message now.

No functional changes to Blender or the build.