Add macro for conditionally choosing code based on Julia version

[simonster]

Some things, like uses of the old FFT API, are difficult/never exported/not used widely enough to define in Compat. However, when it's necessary to select different code based on the Julia version within a function, comparing VERSION directly is a bad idea because it incurs a runtime cost and also potentially screws with type inference. This PR defines a @julia_geq macro that works like:

function f()
    x = @julia_geq v"0.4.0-dev+6632" ? 1 : 2.0
    x + 1
end

It can also be used with blocks:

function f()
    @julia_geq v"0.4.0-dev+6632" ? begin
        ...
    end : begin
        ...
     end
end

I'm open to bikeshedding both the name and the syntax, but since I'm already using something like this in DSP, I figured it might be useful enough to include here.

[simonster]

Also worth referring to JuliaLang/julia#5892 and JuliaLang/julia#7449. In particular, @one-more-minute's macro from JuliaLang/julia#5892 is another possible approach.

[stevengj]

For syntax, how about @VERSION >= v"...."? This should be valid since @VERSION does not conflict with VERSION.

[stevengj]

Of course, it would be better if the compiler could statically eval VersionNumber comparisons, but that seems like another case for JuliaLang/julia#414.

[simonster]

Unfortunately you can't start a macro argument with a comparison operator because they aren't unary operators and so that won't parse, e.g.:

julia> :(>= v"0.4" ? 1 : 2)
ERROR: syntax: ">=" is not a unary operator

[TotalVerb]

This is obsoleted by JuliaLang/julia#16219, which was just merged. Now

@static if VERSION [ ... ] end

can be used.

The @static macro can be provided (and exported) from Compat itself on 0.3 and 0.4.