Handling Operating System Variation

# Handling Operating System Variation

When dealing with platform libraries, it is often necessary to provide special cases for various platforms. The variable `Sys.KERNEL` can be used to write these special cases. There are several functions in the `Sys` module intended to make this easier: `isunix`, `islinux`, `isapple`, `isbsd`, and `iswindows`. These may be used as follows:

    if Sys.iswindows()
        some_complicated_thing(a)
    end

Note that `islinux` and `isapple` are mutually exclusive subsets of `isunix`. Additionally, there is a macro `@static` which makes it possible to use these functions to conditionally hide invalid code, as demonstrated in the following examples.

Simple blocks:

    ccall((@static Sys.iswindows() ? :_fopen : :fopen), ...)

Complex blocks:

    @static if Sys.islinux()
        some_complicated_thing(a)
    else
        some_different_thing(a)
    end

When chaining conditionals (including `if`/`elseif`/`end`), the `@static` must be repeated for each level (parentheses optional, but recommended for readability):

    @static Sys.iswindows() ? :a : (@static Sys.isapple() ? :b : :c)