Towards really cross-toolset testing system.

- Allow to specify toolset name on the command line
- Expand $toolset in paths
- Pass toolset name to bjam invocations.


[SVN r17555]

tools/build

@@ -1 +1 @@
-Subproject commit bfc4c76a241cfacda2696cfbb8d3ccbac0d0046c
+Subproject commit 72a0eb1444c4b8875df87ecd0845344696f2e8eb