Change searched-lib to lib in the Jamfile.

Apparently, searched-lib was not supposed to be used in libraries,
and the simple lib target is the preferred way to declare external
libraries.

bfgroup/b2#431 (comment)

Author: Lastique

build/Jamfile.v2

@@ -17,7 +17,7 @@ import feature ;
 import configure ;
 import atomic-arch-config ;
 
-searched-lib synchronization ;
+lib synchronization ;
 explicit synchronization ;
 
 local cxx_requirements = [ requires