Improve 'flags' and the gcc toolset.

* gcc.jam: Extend the 'toolset' feature with gcc here.
    Use flags for linking too and eliminate 'link-options'
    rule. Remove empty compile/archive/link/link-dll rules.

* toolset.jam (flags): Document somehow.
     (set-target-variables): Call 'actualize' on values
     of dependency features.


[SVN r16440]

tools/build

@@ -1 +1 @@
-Subproject commit b12a902687363924060819268a01f70516569b24
+Subproject commit 0afa3aa911ef7a3d7a10bafa433d272a08ea7e81