Revert "Since HP cc is strict c89, use -std=c89 with gcc."

This reverts commit ff265b04d5aca2feab3937e669b95fe0aefae8cd.

(no way this is going to work with uselargefiles)
fix builds with ExtUtils::ParseXS

A recent change to ExtUtils::ParseXS broke builds: add
    PERL_UNUSED_VAR(file)
only when the 'file' var is present.
Report inaccesible file on failed require

Commit 2433d39e6 (require should die if a file exists but can't be
read) made first failed opened file fatal as request in
[perl #113422]. However error message produced in that case is not
much helpful in identifying which file ound not been accessed:

    $ LANG=C perl -I/root -e 'require strict'
    Can't locate strict.pm:   Permission denied at -e line 1.

This patch adds the name of the failed file to the message to help
identify which @INC directory is erroneous:

    $ LANG=C ./perl -I/root -I./lib -e 'require strict'
    Can't locate strict.pm:   /root/strict.pm: Permission denied at -e line 1.

Signed-off-by: Petr Písař <ppisar@redhat.com>