gegl: fix build from source with meson nofallback

[jonchang]

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Needed for #61226 as a result of Homebrew/brew#8649. Since basically only gegl uses libnsgif and poly2tri-c, they ship the source as part of their distribution tarballs. Re-enable fallbacks for these vendored dependencies since they won't be downloaded separately.

[jonchang]

GEGL is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License and GNU General Public License as published by the Free Software Foundation; either version 3 of the Licenses, or (at your option) any later version. The library itself is licensed under LGPL while the sample commandline application and GUI binary gegl is licensed under GPL. GEGL depends on and ships with the Poly2Tri-C library, which is also free software; you can redistribute it and/or modify it under the terms of the New BSD License.

https://www.gegl.org/copyright.html

libnsgif is MIT licensed: https://gitlab.gnome.org/GNOME/gegl/-/blob/master/subprojects/libnsgif/COPYING

[tschoonj]

LGTM

[BrewTestBot]

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