configure: Add a check for the DGifOpen function in the gif_lib.h header as the v4 of the file does define it, while v3 doesn't.

[Robert Shearman]

---
  configure.ac               |    8 ++++++++
  dlls/oleaut32/olepicture.c |    6 +++---
  2 files changed, 11 insertions(+), 3 deletions(-)
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