This removes all the logic that perl uses to locate an appropriate
errno.h for the target. Instead we simple create a file that does

  #include "errno.h"

and use that as the file to parse. This is needed when using an
external toolchain since perl will search in ${STAGING_INCDIR} for
errno.h (when using gcc) and that isn't where it's located - its
wherever the external toolchain keeps it's headers.

Index: perl-5.10.1/ext/Errno/Errno_pm.PL
===================================================================
--- perl-5.10.1.orig/ext/Errno/Errno_pm.PL	2007-12-18 13:47:07.000000000 +0300
+++ perl-5.10.1/ext/Errno/Errno_pm.PL	2009-08-23 16:10:24.716303050 +0400
@@ -16,8 +16,18 @@
 open OUT, ">Errno.pm" or die "Cannot open Errno.pm: $!";
 select OUT;
 my $file;
-my @files = get_files();
-if ($Config{gccversion} ne '' && $^O eq 'MSWin32') {
+#my @files = get_files();
+my @files = ("errno.h");
+
+if (1) {
+    open INCS, '>includes.c' or
+	die "Cannot open includes.c";
+    print INCS qq[#include "errno.h"\n];
+    close INCS;
+    process_file('includes.c');
+    unlink 'includes.c';
+}
+elsif ($Config{gccversion} ne '' && $^O eq 'MSWin32') {
     # MinGW complains "warning: #pragma system_header ignored outside include
     # file" if the header files are processed individually, so include them
     # all in .c file and process that instead.
@@ -49,7 +59,7 @@
         chomp($file = `cygpath -w "$file"`);
     }
 
-    return unless defined $file and -f $file;
+#    return unless defined $file and -f $file;
 #    warn "Processing $file\n";
 
     local *FH;