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

Works for both GLSL and HLSL.
Fixes #87.

Test/glsl.-D-U.frag

@@ -0,0 +1,32 @@
+#version 450
+
+#define IN_SHADER
+
+layout(location=0) out vec4 color;
+
+void main()
+{
+#if FOO==200
+    color = vec4(1.0);
+#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
+
+#if MUL == 400
+    color *= MUL;
+#else
+    #error bad MUL
+#endif
+}