Corrects a problem in the handling of <conditional>@rule in 
the lib rule, e.g.:

lib foo : : <conditional>@sources ;

Prior to the patch, this was treating the library as a prebuilt
library because of the absence of any source arguments.  Thus
nothing would get built.

 * tools/builtin.jam
   (lib-generator.run): Added additional constraint when setting
   library type to SEARCHED_LIB.