Load optional components only when their build directory is present.

Non-recursive super-project checkout still has empty directory for
all components.

Jamroot

@@ -248,12 +248,12 @@ for local l in $(all-libraries)
     use-project /boost/$(l) : libs/$(l)/build ;
 }
 
-if [ path.exists $(BOOST_ROOT)/tools/inspect ]
+if [ path.exists $(BOOST_ROOT)/tools/inspect/build ]
 {
     use-project /boost/tools/inspect : tools/inspect/build ;
 }
 
-if [ path.exists $(BOOST_ROOT)/libs/wave/tool ]
+if [ path.exists $(BOOST_ROOT)/libs/wave/tool/build ]
 {
     use-project /boost/libs/wave/tool : libs/wave/tool/build ;
 }