Added arbitrary flag support to try_expand()

[x87-va]

It's needed to pass macOS specific flags. For example: -F/Library/Frameworks.

[alexcrichton]

As-is this is passing flags for ar, not cc, so I don't think this may be what you're intending?

[x87-va]

From documentation for ar_flag: "Add an arbitrary flag to the invocation of the compiler". For example: I need to expand some macros for framework located in /Library/Frameworks:

    let src = "expando.c";
    let mut gcc = cc::Build::new();
    gcc.ar_flag("-F/Library/Frameworks");
    let expanded = gcc.file(src).try_expand().expect("Expand");

[alexcrichton]

I believe that's a copy/paste error on the ar_flag method, that only goes to the ar invocation, not the compiler itself.

[x87-va]

Ok, I've fixed it.

[alexcrichton]

Thanks for the doc update, but I don't think that the other changes are necessary here. The contents of self.flags should already be part of the command invocation of try_get_compiler.

[x87-va]

Ok, I checked this. It's really unnecessary changes.

[alexcrichton]

Thanks!