Implement --keep-going option.

[SVN r57979]

v2/build-system.jam

@@ -904,6 +904,21 @@ local rule should-clean-project ( project )
         modules.poke : PARALLELISM : $(j) ;
     }
     
+    local k = [ option.get keep-going : false : true ] ;
+    if $(k) in "on" "yes" "true"
+    {
+        modules.poke : KEEP_GOING : 1 ;
+    }
+    else if $(k) in "off" "no" "false"
+    {
+        modules.poke : KEEP_GOING : 0 ;
+    }
+    else
+    {
+        ECHO "error: Invalid value for the --keep-going option" ;
+        EXIT ;
+    }
+                
     # The 'all' pseudo target is not strictly needed expect in the case when we
     # use it below but people often assume they always have this target
     # available and do not declare it themselves before use which may cause