Force toolset names to lowercase. Fixes problems with case insensitive fs where it doesn't fail to find the toolset if the case is different, but produces cased results which are incompatible with other builds.

[SVN r22355]

tools/build

@@ -1 +1 @@
-Subproject commit b8ae5d7b8ad5591b3e116b54fd60c744459ba1ff
+Subproject commit 68bfe6aa2e8475d6ceebc5a6abb662ce8a3ce56d