Work around a header name collission on util.h

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.

Makefile.PL

@@ -8,6 +8,7 @@ if ( $^O eq 'MSWin32' ) {
 
 use strict;
 use IO::File;
+use File::Spec;
 use Config qw(%Config);
 
 my %cfg;
@@ -203,6 +204,33 @@ ESQ
   else {
     $headers{$h} = undef;
     $define{"-DHAVE_\U$def"} = $h;
+    if ( $h eq 'util.h' ) {
+        # Jump through hoops due to a header clash collision with perl
+        # The following is highly unportable.
+
+        # First, we need to figure out where the C compiler is looking
+        # for includes.
+        my $raw_cc_output = qx($cfg{'cc'} $flags -E -Wp,-v -xc /dev/null 2>&1);
+        my @cc_output = split /\n+/, $raw_cc_output;
+        my @inc_paths;
+        foreach my $maybe_inc_path ( @cc_output ) {
+            next unless $maybe_inc_path =~ /\A\s+/;
+            my (undef, $inc_path) = split /\s+/, $maybe_inc_path, 3;
+            push @inc_paths, $inc_path;
+        }
+
+        # With the list of include directories, try to find util.h
+        foreach my $inc_path ( @inc_paths ) {
+            my $abs_header_path = File::Spec->catfile($inc_path, 'util.h');
+            next unless -e $abs_header_path;
+            # Bingo!  Now we need to let the C compiler know, so that our XS
+            # file will include it.
+            # Again massively non-portable -- we ideally should be using something
+            # smart to quote the value.
+            $define{qq<-DUTIL_H_ABS_PATH=\\"$abs_header_path\\">} = $h if $abs_header_path;
+            last;
+        }
+    }
     print "FOUND.\n";
     unlink "headtest_$def.c", "headtest_$def.log";
   }

Tty.xs

@@ -58,7 +58,9 @@ typedef FILE * InOutStream;
 #endif /* HAVE_UTIL_H */
 
 #ifdef HAVE_UTIL_H
-# if ((PATCHLEVEL < 19) && (SUBVERSION < 4))
+# ifdef UTIL_H_ABS_PATH
+#  include UTIL_H_ABS_PATH
+# elif ((PATCHLEVEL < 19) && (SUBVERSION < 4))
 #  include <util.h>
 # endif
 #endif /* HAVE_UTIL_H */