Work around a header name collission on util.h

[Hugmeir]

Perl provides a util.h.
openpty() comes from a system util.h in OSX.

The end result is that the system util.h is unreachable,
and so openpty() is never declared.

This commit works around this in a nasty and non-portable
fashiom. First, it adds a second way of including the header
in the XS code:

#ifdef UTIL_H_ABS_PATH
# define UTIL_H_ABS_PATH
#endif

And second, it uses an arcane command to figure out
the exact header that gcc or clang ended up finding.

It's pretty terrible, all things told, but since it'll only
really kick in for OSX, the horror is somewhat contained.

This was previously reported here: #25

[toddr]

@haarg indicates that: "upgrading EUMM should allow IO::Tty to work". Can you clarify what version? should we require it or is that not worth it given it's only an OSX issue?

[haarg]

The new XCode has enabled errors for "implicit declaration of function", which is why this started failing. ExtUtils::MakeMaker 7.49_04 works around this by disabling those errors.

But the util.h problem is still worth addressing IMO. The workaround in this PR is ugly, but I'm not aware of any easy fixes.

It may be worth addressing this in perl core. Having a util.h header is rather unfortunate, and likely could be renamed.