Add variants - #137

Add build script methods: variant, variant_group, with_variants

build_tests/variants/Rsconscript

@@ -0,0 +1,13 @@
+variant "debug"
+variant "release"
+
+with_variants do
+  env "prog" do |env|
+    if variant("debug")
+      env["CPPDEFINES"] << "DEBUG"
+    else
+      env["CPPDEFINES"] << "NDEBUG"
+    end
+    env.Program("^/prog.exe", "prog.c")
+  end
+end