Log ext lib build steps to provide user feedback.
Add package_ext_libs_finish.sh script to separate packaging
libraries from building them, to avoid build database conflicts.
Travis only build macOS ext libs.
Add ExternalDependencies (Debug) scheme to ExternalDependencies Xcode project.
Add --debug option to fetchDependencies script.
Support Release, Debug & Latest directories in External/build directory.
Enable DEPLOYMENT_POSTPROCESSING build setting when compiling
SPIRV-Tools to avoid warning spam when building dylibs.
Disable visibility warnings when building MoltenVKShaderConverter
in Release mode from external libraries built in Debug mode.
Always use -Xlinker -w option when creating dylibs to disable visibility warnings.
The fetchDependencies script puts build files in External/build directory.
Xcode puts build files in standard Library/.../DerivedData directory.
Copy compiled external static library files to External/build/iOS &
External/build/macOS regardless of location of intermediates.
