pass extra variables into Environment.execute()

lib/rscons/builders/cc.rb

@@ -23,7 +23,11 @@ module Rscons
           '-o', o_file,
           source
         ]
-        env.execute("CC #{o_file}", command)
+        vars = {
+          'TARGET' => o_file,
+          'SOURCES' => source,
+        }
+        env.execute("CC #{o_file}", command, vars)
         Cache.open.register_build(target, [source])
       end
       o_file

lib/rscons/builders/program.rb

@@ -34,7 +34,11 @@ module Rscons
           *env['LIBPATHS'].map {|lp| "-L#{lp}"},
           *env['LIBS'].map {|lib| "-l#{lib}"}
         ]
-        env.execute("LINK #{target}", command)
+        vars = {
+          'TARGET' => target,
+          'SOURCES' => sources,
+        }
+        env.execute("LINK #{target}", command, vars)
         Cache.open.register_build(target, sources)
       end
       target

lib/rscons/environment.rb

@@ -47,7 +47,8 @@ module Rscons
       @build_dirs[src_dir] = build_dir
     end
 
-    def execute(short_desc, command)
+    def execute(short_desc, command, extra_vars)
+      merged_variables = @variables.merge(extra_vars)
       expand_varref = proc do |varref|
         if varref.is_a?(Array)
           varref.map do |ent|
@@ -56,7 +57,7 @@ module Rscons
         else
           if varref[0] == '$'
             varname = varref[1, varref.size]
-            varval = self[varname]
+            varval = merged_variables[varname]
             if varval
               expand_varref.call(varval)
             else