Update to rscons 3.1.0

.gitignore

@@ -1,4 +1,2 @@
-/app
-/app.exe
 /build/
 /.rscons*

Rsconscript

@@ -3,30 +3,28 @@ configure do
   check_cfg package: "freetype2"
 end
 
-build do
-  Environment.new do |env|
-    sources = []
-    dirs = [
-      "src",
-      "modules/DerelictUtil/source",
-      "modules/DerelictSDL2/source",
-      "modules/DerelictGL3/source",
-      "modules/gl3n/gl3n",
-      "modules/glamour/glamour",
-      "modules/dft/src",
-    ].each do |dir|
-      sources += glob("#{dir}/**/*.{d,c}")
-    end
-    env["D_IMPORT_PATH"] += [
-      "modules/DerelictUtil/source",
-      "modules/DerelictSDL2/source",
-      "modules/DerelictGL3/source",
-      "modules/gl3n",
-      "modules/glamour",
-      "modules/dft/src",
-    ]
-    env["DFLAGS"] += ["-fversion=Derelict3", "-fversion=gl3n", "-fversion=SDLImage2"]
-    env["LDFLAGS"] += ["-static-libgcc"]
-    env.Program("app", sources)
+env "app" do |env|
+  sources = []
+  dirs = [
+    "src",
+    "modules/DerelictUtil/source",
+    "modules/DerelictSDL2/source",
+    "modules/DerelictGL3/source",
+    "modules/gl3n/gl3n",
+    "modules/glamour/glamour",
+    "modules/dft/src",
+  ].each do |dir|
+    sources += glob("#{dir}/**/*.{d,c}")
   end
+  env["D_IMPORT_PATH"] += [
+    "modules/DerelictUtil/source",
+    "modules/DerelictSDL2/source",
+    "modules/DerelictGL3/source",
+    "modules/gl3n",
+    "modules/glamour",
+    "modules/dft/src",
+  ]
+  env["DFLAGS"] += ["-fversion=Derelict3", "-fversion=gl3n", "-fversion=SDLImage2"]
+  env["LDFLAGS"] += ["-static-libgcc"]
+  env.Program("^/app", sources)
 end