[gold] RFC: Add options for controlling mmap and fallocate of output file

[Cary Coutant]

>> Don't I still need to include <linux/falloc.h> and then call
>> ::fallocate under an #ifdef HAVE_LINUX_FALLOC_H? The only declaration
>> for fallocate() I found in the Linux headers is in that header file.
>> Since the man page said it's highly non-portable, I figured I needed
>> the configure test.
>
> At least on my system <linux/falloc.h> doesn't declare fallocate
> anyhow.  It is declared by <fcntl.h>, though.

Ah, right -- it's in bits/fcntl.h. OK, so I don't need the configure
test for <linux/falloc.h>, but do I need a configure test for
fallocate itself, or can I just assume it's there?

-cary