[parrot/parrot] 0de510: When pkg-config not detected, set to 0.

[GitHub]

  Branch: refs/heads/840/has_libffi
  Home:   https://github.com/parrot/parrot
  Commit: 0de5107f3f6d2fea3160486a18ff8a827f72c690
      https://github.com/parrot/parrot/commit/0de5107f3f6d2fea3160486a18ff8a827f72c690
  Author: jkeenan <jkeenan at cpan.org>
  Date:   2012-09-23 (Sun, 23 Sep 2012)

  Changed paths:
    M config/auto/libffi.pm
    M t/steps/auto/libffi-01.t

  Log Message:
  -----------
  When pkg-config not detected, set to 0.

For consistency with other header detectors, set HAS_LIBFFI to 0 rather than undef.

For #840.