Handling for base type generators fixes.

* new/generators.jam
     (find-viable-generators): Reverted my revert. Not yet sure
         what's the best way, but this one can be made to work.
     (try-one-generators): Adjust the virtual target types in
         case generator for base type was used.

* new/virtual-target.jam (virtual-target.set-type): New method.


[SVN r16234]

tools/build

@@ -1 +1 @@
-Subproject commit 4bb989010711792f3e6286bc1104947d22e4abde
+Subproject commit c4772e11f2dd0a72de89e629b072f6edb8f5e967