Standalone: Implement -D and -U for preprocessor macros.

Works for both GLSL and HLSL.
Fixes #87.

Test/hlsl.-D-U.frag

@@ -0,0 +1,31 @@
+
+#define IN_SHADER
+
+static float4 color;
+
+void main()
+{
+#if FOO==200
+    color = 1.0;
+#else
+    #error expected FOO 200
+#endif
+
+#ifdef FOO
+    color -= 5.0;
+#else
+    #error expected FOO 200
+#endif
+
+#ifdef IN_SHADER
+    color++;
+#else
+    #error IN_SHADER was undef
+#endif
+
+#ifdef UNDEFED
+    #error UNDEFED defined
+#else
+    color *= 3.0;
+#endif
+}