Added SKIP block for bug in OpenBSD's pkg-config

- OpenBSD replaced pkg-config with a Perl script; the Perl script in
  OpenBSD 5.2 and earlier does not honor the --max-version pkg-config
  flag; SKIP tests that work with --max-version on OpenBSD and with
  pkg-config version 0.26 or earlier
Updated Makefile.PL to use META_MERGE block

- The output from META_MERGE is what populates all the nice bits on
  MetaCPAN
Stable release 1.14
Empty cflags are valid cflags

ExtUtils::PkgConfig->find() croaked on emptry cflags:

$ pkg-config --cflags 'libffi >= 3.0.0'| hexdump -C
00000000  0a                                                |.|
00000001
$ perl -MData::Dumper -MExtUtils::PkgConfig -e 'print Dumper(ExtUtils::PkgConfig->find(q{libffi}))'
*** can't find cflags for "libffi"
*** is it properly installed and available in PKG_CONFIG_PATH?
 at -e line 1.

That's wrong. Empty cflags are valid cflags too.

<https://rt.cpan.org/Public/Bug/Display.html?id=84026>

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