build: CMakeLists.txt cleanup, part 1

build: CMakeLists.txt cleanup, part 1

This is a first pass reorganization of CMake files in this repo.
It consists primarily of dead code/variable removal, simplification,
and reformatting by latest cmake-format.

bump to cmake_minimum_required(3.4), for ccache support.

add USE_CCACHE

change 'ln -sf' to '-E create_symlink'

clarify why CMAKE_OSX_DEPLOYMENT_TARGET is pre-project().

sync FindVulkan.cmake from upstream CMake v3.8.0
  Ensure our repos use identical copy of upstream FindVulkan.cmake
  Copied from Modules/FindVulkan.cmake
  https://gitlab.kitware.com/cmake/cmake.git
  tag: v3.8.0 (commit da7833c5bb1f331162d46a2c664a443c1c641089)

change $<CONFIGURATION> to $<CONFIG>
  The $<CONFIGURATION> genexp is officially deprecated.

change CMAKE_SYSTEM_NAME to UNIX/APPLE/WIN32
  Note that UNIX evaluates true for OSX, so whenever the code intends
  "Linux only", we use (UNIX AND NOT APPLE).

change TOOLS_TARGET_FOLDER to TOOLS_HELPER_FOLDER
  for consistency with VVL and VL repos.

set Windows install prefix if needed

remove unused DisplayServer variable

remove deprecated GLSLANG_REPO_ROOT
  The *_REPO_ROOT mechanism for finding packages is deprecated.

remove extra cmake_minimum_required

change LIBVK to Vulkan::Vulkan

change PYTHON_CMD to PYTHON_EXECUTABLE

section rulers, other cosmetics

.cmake-format.py 0.4.1

reformat using cmake-format 0.4.1

Change-Id: Id8e5b26fdcf5dc0b383de94cbec75a567704a55e