Merger main Boost library Jamroot file changes from the trunk. [Added a missing import of the Boost Build generate.jam module. Although not strictly necessary at the moment this will avoid future build breakage when Boost Build removes some of its extra internal module imports.]

[SVN r47685]

Jamroot

@@ -98,6 +98,7 @@
 #  - handle boost version
 #  - handle python options such as pydebug
 
+import generate ;
 import modules ;
 import set ;
 import stage ;