For example, this would allow compilers to add generated headers into include
path.
* virtual-target.jam (action.adjust-properties): New rule.
(clone-action-template): Don't slice cloned object.
* generators.jam (generator.action-class): New rule, to allow
overriding action class on per-generator basis.
* builtin.jam (compile-action): New class.
(C-compiling-generator): New class.
* gcc.jam: Use C-compiling-generator for gcc. Honor includes.
[SVN r15618]
