AIX does not have any of the BSD's warnx and friends functions, and neither
does it have getopt_long. Both of these are included, sourced from a pool of
permissive licenses, with their license notices intact.
In an effort to preserve nativity, this code is not used on other platforms,
instead preferring to use the native behaviours available on respective
platforms.
Additionally, this commit also includes some changes that _removes_
platform-specificity for the Linux platform, setting the _GNU_SOURCE macro
once in autoconf, instead of duplicating that in every source file.
Signed-off-by: Akash Kulhalli <akash.kulhalli@ibm.com>
