build: CMakeLists cleanup and modernization

Includes: using modern CMake idioms, removing dead code/variables,
simplifying conditionals, and some cosmetic/whitespace changes.

Apply warning flag only the appropriate compiler

Remove unneeded cmake_minimum_required()

Make platform-specific if/then trees consistent.  Use CMake's platform
specifier variables with a consistent conditional tree structure.

Remove unused MAJOR variable

Clean declaration and use of API_NAME variable

Add PRIVATE to limit scope of add target_link_libraries()

Inline the unneeded run_external_revision_generate() macro

Remove redundant VkLayer_utils declaration

Remove redundant add_dependencies()

Rename macro from run_vk_xml_generate to GenerateFromVkXml.
Rename macro from add_vk_layer to AddVkLayer
Rename macro add_test_layer to AddVkLayer
For consistency with Vulkan-Loader repo, and at least notionally with
the C++ style guide, use camel-case names for macros.

Move macro invocations closer to macro definition

Rename variable LAYER_JSON_FILES to TEST_LAYER_NAME.
Use better name for variable containing a single object.

Use varialble TEST_LAYER_NAME to remove repetition.
Avoid many reuses of explicit name by using variable.

Remove deprecated use of CMAKE_{C|CXX}_FLAGS

Use modern target_compile_definitions() and target_compile_options()

Use target_include_directories()

Replace deprecated include_directories() with modern construct.

Remove unneeded foreach() over a single element.

Replace deprecated set_target_properties() with target_compile_options()

Add vk_safe_struct.cpp as a generated dependency.

Rework/simplify setting of compiler flags and standards.

Cosmetics (comments, rulers, whitespace)

Reformat with cmake_format v0.4.1

Change-Id: I9b2ed43c09030b44f6a779dc2243464f45ce5e2a