This tries to load vulkan.framework or libvulkan.1.dylib before MoltenVK.framework
or libMoltenVK.dylib. In the previous version, layers would not work for applications
run-time loading the default library.
SDL now builds with gcc 7.2 with the following command line options:
-Wall -pedantic-errors -Wno-deprecated-declarations -Wno-overlength-strings --std=c99