When inheriting generators, allow to replace original <toolset> property

with arbitrary property list, for example

     <toolset>intel <toolset-intel:platform>win


[SVN r22928]

tools/build

@@ -1 +1 @@
-Subproject commit 3e37e47c8d2cffcef826106f28ac5960140d92f1
+Subproject commit 7ef073ddc5d8c9a5f2bd24a5e3f9e11ad38ea2a3