gimp: reenable avif/heic
libheif commit cf0d89c6e0809427427583290547a7757428cf5a removes the definitions for the builtin_h265_decoder, builtin_h265_encoder, builtin_avif_decoder, and builtin_avif_encoder pkg-config variables. This commit is included in libheif 1.80.0 and newer. gimp 2.10.38 depends on these pkg-config variables in its autoconf morass, and as a result is built without support for importing or exporting avif and heic. This commit "fixes" this by forcefully setting these feature detection flags to "yes" if any version of libheif is present. Technically this logic should only apply to libheif 1.80.0 and above, but I don't think there's really any good reason to bother with that additional complication for a patch meant specifically for compiling gimp in a rolling release distribution.
Some links:
- https://www.github.com/strukturag/libheif/issues/758
- https://gitlab.gnome.org/GNOME/gimp/-/issues/9080
Testing the changes
- I tested the changes in this PR: YES
Local build testing
- I built this PR locally for my native architecture, (x86_64-glibc)