Per subproject default_library option

The default_library built-in option can now be defined per subproject. This is useful for example when building shared libraries in the main project, but static link a subproject.

Most of the time this would be used either by the parent project by setting subproject's default_options (e.g. subproject('foo', default_options: 'default_library=static')), or by the user using the command line -Dfoo:default_library=static.

The value is overriden in this order:

• Value from parent project
• Value from subproject's default_options if set
• Value from subproject() default_options if set
• Value from command line if set