Set CMAKE_POLICY_VERSION_MINIMUM=3.9 to allow IPO with gcc

Author: fdintino

.github/workflows/wheels-dependencies.sh

@@ -132,8 +132,10 @@ function build_libavif {
     # libavif) to disable the compilation and inclusion of aom's AV1 decoder.
     # CONFIG_AV1_HIGHBITDEPTH=0 is another flag for libaom that disables support
     # for encoding high bit depth images.
+    # CMAKE_POLICY_VERSION_MINIMUM=3.9 enables cmake policy CMP0069, which is
+    # required to allow INTERPROCEDURAL_OPTIMIZATION (-flto) WITH gcc
     (cd $out_dir \
-        && CMAKE_POLICY_VERSION_MINIMUM=3.5 cmake \
+        && CMAKE_POLICY_VERSION_MINIMUM=3.9 cmake \
             -DCMAKE_INSTALL_PREFIX=$BUILD_PREFIX \
             -DCMAKE_INSTALL_LIBDIR=$BUILD_PREFIX/lib \
             -DCMAKE_INSTALL_NAME_DIR=$BUILD_PREFIX/lib \