gifdec isn't part of imageio lib, it's only used by gif2webp.
Change-Id: I70bff378a32f8fb2ebb8a5a7701049ffff7f7992
(cherry picked from commit dcbc1c881a001f45081834a47391d673c1b96907)
This will well isolate contributions for original code,
generated code and SIMD (especially for Android).
Change-Id: Ie47664decc7f43c2f57260a72cab951c347281a7