Implement 'explicit targets'. This closes BB29, except for documentation

issues.

* new/project.jam: New project attribute 'explicit-targets'.
* new/targets.jam (project-target): Don't implicitly generate targets
  that are in 'explicit-targets' attribute.


[SVN r18294]

tools/build

@@ -1 +1 @@
-Subproject commit 90b7e4600d8f0149d04e7acbeafe1949c90bee03
+Subproject commit bacc81dfc1f7a0f008bebd04f304718842f4fb81