Modules that use extensions

Using standard derivatives is a common way to anti-alias 2D shapes, but it requires an extension. What's the best way of modularizing something that looks like this?

``` glsl
#ifdef GL_OES_standard_derivatives
#extension GL_OES_standard_derivatives : enable
#endif

float aastep(float threshold, float value) {
#ifdef GL_OES_standard_derivatives
float afwidth = 0.7 * length(vec2(dFdx(value), dFdy(value)));
#else
float afwidth = /* some fallback approach */;
#endif
return smoothstep(threshold-afwidth, threshold+afwidth, value);
}
```

More reading:
http://webstaff.itn.liu.se/~stegu/webglshadertutorial/shadertutorial.html (part 3)
https://github.com/Jam3/three-bmfont-text/issues/1