Cleanups. Default build improvements related to BB10.

* new/targets.jam
    (main-target.default-build): New field.
    (main-target.add-alternative): Don't allow default-build for
       second and subsequence alternatives. Record default build
       from the first one.
    (main-target.generate): Expand build request.
    (main-target.generate-really): New rule, extracted from 'generate'.
    (basic-target.default-build): New rule.
    (basic-target.generate): Don't expand default build.

* test/BoostBuild.py
  Allow to suppress passing toolset when calling run_build_system.
  Some Python 2.1 compatibility fixes.

* test/alternatives.py
  More tests.

* test/default_build.py
  More tests.


[SVN r17683]

tools/build

@@ -1 +1 @@
-Subproject commit 3acedf660f40159fe7de298f9d01ce6487798a9b
+Subproject commit 5aeb1de1919e252f48b3e3999c5b2a52b5d04164