add Rscons::BuildTarget

Return a BuildTarget object from Environment#method_missing when a build target is registered.

spec/rscons/builders/program_spec.rb

@@ -5,13 +5,13 @@ module Rscons
       subject {Program.new}
 
       it "supports overriding CC construction variable" do
-        subject.should_receive(:standard_build).with("LD prog#{env["PROGSUFFIX"]}", "prog#{env["PROGSUFFIX"]}", ["sp-c++", "-o", "prog#{env["PROGSUFFIX"]}", "prog.o"], ["prog.o"], env, :cache)
+        subject.should_receive(:standard_build).with("LD prog", "prog", ["sp-c++", "-o", "prog", "prog.o"], ["prog.o"], env, :cache)
         subject.run("prog", ["prog.o"], :cache, env, "CC" => "sp-c++")
       end
 
       it "supports overriding LDCMD construction variable" do
         subject.should_receive(:standard_build).with("LD prog.exe", "prog.exe", ["special", "LD!", "prog.o"], ["prog.o"], env, :cache)
-        subject.run("prog", ["prog.o"], :cache, env, "LDCMD" => ["special", "LD!", "${_SOURCES}"])
+        subject.run("prog.exe", ["prog.o"], :cache, env, "LDCMD" => ["special", "LD!", "${_SOURCES}"])
       end
     end
   end