Move inter-lib dependencies to a project variable and into the build targets.

build.jam

@@ -5,11 +5,13 @@
 
 require-b2 5.2 ;
 
+constant boost_dependencies :
+    /boost/config//boost_config
+    /boost/io//boost_io
+    /boost/predef//boost_predef ;
+
 project /boost/timer
     : common-requirements
-        <library>/boost/config//boost_config
-        <library>/boost/io//boost_io
-        <library>/boost/predef//boost_predef
         <include>include
     ;
 
@@ -21,3 +23,4 @@ explicit
 call-if : boost-library timer
     : install boost_timer
     ;
+

build/Jamfile.v2

@@ -9,6 +9,9 @@
 
 SOURCES = auto_timers_construction cpu_timer ;
 
+project
+    : common-requirements <library>$(boost_dependencies) ;
+
 lib boost_timer
    : ../src/$(SOURCES).cpp
    : # requirements