graphics/libavif: fix build with clang 14

Clang >= 14 warns that mixing declarations and code is incompatible with
standards before C99, even if you compile with -std=c99 or -std=gnu99.

Add -Wno-declaration-after-statement if it is supported, to silence
the warning.

PR: 262634
Approved by: jbeich (maintainer)
Obtained from: https://github.com/AOMediaCodec/libavif/pull/886
MFH: 2022Q1