diff --git a/src/build/toolset.jam b/src/build/toolset.jam
index 5af9c6842..21a23fd6d 100644
--- a/src/build/toolset.jam
+++ b/src/build/toolset.jam
@@ -7,6 +7,8 @@
 
 import feature ;
 import numbers ;
+import errors : error ;
+import property ;
 
 .flag-no = 1 ;
 
@@ -42,6 +44,9 @@ rule flags ( rule-or-module   # If contains dot, should be a rule name.
                               # - a set of property sets. If one of those property 
                               #   sets is contained in build properties, 'values'
                               #   will be added to the variable.
+                              #
+                              # - empty. 'values' will be added to the variable 
+                              #   unconditionally
              values * )
 {
     .$(rule-or-module).flags += $(.flag-no) ;
@@ -50,58 +55,103 @@ rule flags ( rule-or-module   # If contains dot, should be a rule name.
     .$(rule-or-module).condition.$(.flag-no) += $(condition) ;    
     .$(rule-or-module).values.$(.flag-no) += $(values) ;
     
+    if $(condition[1]:G=)
+    {
+        .$(rule-or-module).match-type.$(.flag-no) += property-set ;
+        property.validate-property-sets $(condition) ;
+        if ! $(values)
+        {
+            error empty value set used with property-set match criterion:
+              : $(condition) ;
+        }
+    }
+    else if $(condition)
+    {
+        .$(rule-or-module).match-type.$(.flag-no) += feature ;
+        if $(values)
+        {
+            error non-empty value set used with feature match criterion
+        }
+    }
+    else
+    {
+        .$(rule-or-module).match-type.$(.flag-no) += unconditional ;
+        error empty value set used with unconditional match criterion ;
+    }
+    
     .flag-no = [ numbers.increment $(.flag-no) ] ;
 }
 
+# Returns the first element of 'property-sets' which is a subset of
+# 'properties', or an empty list if no such element exists.
+local rule find-property-subset ( property-sets * : properties * )
+{
+    local result ;
+    for local s in $(property-sets)
+    {
+        if ! $(result)
+        {
+            if [ feature.split $(s) ] in $(properties)
+            {
+                result = $(s) ;
+            }
+        }
+    }
+    return $(result) ;
+}
 
 rule set-target-variables ( rule-or-module target : properties * )
-{    
+{ 
     for local f in $(.$(rule-or-module).flags) 
     {
         local variable = $(.$(rule-or-module).variable.$(f)) ;
         local condition = $(.$(rule-or-module).condition.$(f)) ;
         local values = $(.$(rule-or-module).values.$(f)) ;
         local result ;
-        local found ;
-                
-        for local c in $(condition) 
+        
+        switch $(.$(rule-or-module).match-type.$(f))
         {
-            local requirements = [ feature.split $(c) ] ;            
-                        
-            if $(requirements:G=)
+         case unconditional :
+              result += $(values) ;
+              
+         case property-set : 
+            if [ find-property-subset $(condition) : $(properties) ]
             {
-                # This is a property set
-                if ! $(found) && $(requirements) in $(properties)
+                result += $(values) ;
+            }
+            
+         case feature : # add the values of all specified features to the variable
+            local matches = [ property.select $(condition) : $(properties) ] ;
+            for local p in $(matches)
+            {
+                if dependency in [ feature.attributes $(p:G) ]
                 {
-                    result += $(values) ;
-                    found = 1 ;
-                }                
+                    # the value of a dependency feature is a target
+                    # and must be actualized
+                    result += [ $(p:G=).actualize ] ;
+                }
+                else
+                {
+                    result += $(p:G=) ;
+                }
             }
-            else
-            {
-                for local r in $(requirements)
-                { 
-                    local item = [ feature.get-values $(r) : $(properties) ] ;
-                    for local i in $(item)
-                    {                        
-                        if dependency in [ feature.attributes $(r:G) ] 
-                        {
-                            result += [ $(i).actualize ] ;
-                        }
-                        else
-                        {
-                            result += $(i) ;
-                        }
-                    }                    
-                }                
-            }
-        }
-        if ! $(condition)
-        {
-            result = $(values) ;
         }
 
         $(variable) on $(target) += $(result) ;
     }
+    
+    # recurse for any module-specific flags
+    local module = [ MATCH ^(.+)\\..* : $(rule-or-module) ] ;
+    if $(module)
+    {
+        set-target-variables ( $(module) $(target) : $(properties) * ) ;
+    }
 }
 
+local rule __test__ ( )
+{
+    import assert ;
+    local p = <c>1 <d>2 <e>3 ;
+    assert.result <c>1 <d>2 : find-property-subset <a>0/<b>0/<c>1/<d>2/<e>5 <a>9 : $(p) ;
+    assert.result : find-property-subset <a>0/<b>0/<c>9/<d>9/<e>5 <a>9 : $(p) ;
+}
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