CMake/macOS: Remove _LIBPATH, avoid link_directories.

After tests were bundled in a single executable and cycles and libmv
created their own tests, the warnings on macOS have gone over 800.
The reason is setting `*_LIBRARIES` to names of the libraries
and later using `link_directories` to link them properly.

https://cmake.org/cmake/help/latest/command/link_directories.html

> Note This command is rarely necessary and should be avoided where
> there are other choices. Prefer to pass full absolute paths to
> libraries where possible, since this ensures the correct library
> will always be linked. The find_library() command provides the
> full path, which can generally be used directly in calls to
> target_link_libraries().

Warnings like the following popup for every target/executable,
for every library it links to.
```
ld: warning: directory not found for option
'-L/Users/me/blender-build/blender/../lib/darwin/jpeg/lib/Debug'
```

The patch completes a step towards removing `link_directories` as
mentioned in TODO at several places.
The patch uses absolute paths to link libraries and removes
all `*_LIBPATH`s except `PYTHON_LIBPATH` from
`platform_apple.cmake` file. (The corner case where it's used seems
like dead code. Python is no longer shipped with that file structure.)

Also, unused code for LLVM-3.4 has been removed.
Also, guards to avoid searching libraries in system directories have
been added.

`APPLE` platform now no longer needs `setup_libdirs`,
`cycles_link_directories`, and `link_directories`.
The number of warnings now is less than 100, most of them being
deprecation ones in dependencies.

This patch depended on {rBb746179d0add}, {rB2fdbe4d05011},
{rB402a4cadba49} and {rBd7f482f88ecb}.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D8855

source/creator/CMakeLists.txt

@@ -73,7 +73,10 @@ endif()
 
 if(WITH_TBB)
   blender_include_dirs(${TBB_INCLUDE_DIRS})
-  link_directories(${LIBDIR}/tbb/lib)
+  if(NOT APPLE)
+    # APPLE plaform uses full paths for linking libraries.
+    link_directories(${LIBDIR}/tbb/lib)
+  endif()
 endif()