COMP: SlicerExecutionModelConfig: Set default variable only if not defined

It is now possible for a project to globally override the default value
of output directories and library cxx wrapper.
Merge branches 'add-option-to-skip-cli-shared-libs-install', 'macosx-link-executable-with-rpath' and 'config-set-default-vars-only-if-needed' into pu

* add-option-to-skip-cli-shared-libs-install:
  COMP: Add global option allowing to skip install of CLI shared library

* macosx-link-executable-with-rpath:
  Add option allowing to specify which link flags to use with CLI executable

* config-set-default-vars-only-if-needed:
  COMP: SlicerExecutionModelConfig: Set default variable only if not defined

Conflicts:
	SlicerExecutionModelConfig.cmake.in