Switch Boost.System to header-only in b2

Author: pdimov

build.jam

@@ -16,10 +16,9 @@ constant boost_dependencies :
 project /boost/system ;
 
 explicit
-    [ alias boost_system : build//boost_system ]
+    [ alias boost_system : : : : <include>include <library>$(boost_dependencies) ]
     [ alias all : boost_system test ]
     ;
 
 call-if : boost-library system
-    : install boost_system
     ;