Resolve confict between builtin.lib-generator and $(toolset).prebuilt

in favour of the latter (toolset.prebuilt). This cuts down the number
of generator invocations for prebuilt target and is more clear.

Thanks to Mark Evans for pointing this out.


[SVN r32952]

tools/build

@@ -1 +1 @@
-Subproject commit 776f6adfbf3c09e8a3cbdb7098a9d66cadbdb867
+Subproject commit d2f9daebca21d16b6049da6677a652fa423e7d5c