ext/avm.cmd is broken

[wantehchang]

Although ext/avm.cmd builds AVM successfully, the libavif build fails when linking the libavif.so shared library:

[1/51] Linking CXX shared library libavif.so.16.2.1
FAILED: libavif.so.16.2.1
: && /usr/bin/c++ -fPIC -g -Wl,--dependency-file=CMakeFiles/avif.dir/link.d -shared -Wl,-soname,libavif.so.16 -o libavif.so.16.2.1 CMakeFiles/avif_obj.dir/src/alpha.c.o CMakeFiles/avif_obj.dir/src/avif.c.o CMakeFiles/avif_obj.dir/src/colr.c.o CMakeFiles/avif_obj.dir/src/colrconvert.c.o CMakeFiles/avif_obj.dir/src/diag.c.o CMakeFiles/avif_obj.dir/src/exif.c.o CMakeFiles/avif_obj.dir/src/gainmap.c.o CMakeFiles/avif_obj.dir/src/io.c.o CMakeFiles/avif_obj.dir/src/mem.c.o CMakeFiles/avif_obj.dir/src/obu.c.o CMakeFiles/avif_obj.dir/src/properties.c.o CMakeFiles/avif_obj.dir/src/rawdata.c.o CMakeFiles/avif_obj.dir/src/read.c.o CMakeFiles/avif_obj.dir/src/reformat.c.o CMakeFiles/avif_obj.dir/src/reformat_libsharpyuv.c.o CMakeFiles/avif_obj.dir/src/reformat_libyuv.c.o CMakeFiles/avif_obj.dir/src/scale.c.o CMakeFiles/avif_obj.dir/src/stream.c.o CMakeFiles/avif_obj.dir/src/utils.c.o CMakeFiles/avif_obj.dir/src/write.c.o CMakeFiles/avif_obj.dir/src/codec_avm.c.o _deps/libyuv-build/libyuv.a -lm /usr/local/google/home/wtc/libavif.3/libavif/ext/avm/build.libavif/libaom.a -l_aom_dep_lib_avm-NOTFOUND -lm /usr/lib/x86_64-linux-gnu/libpthread.a && :
/usr/bin/ld: cannot find -l_aom_dep_lib_avm-NOTFOUND: No such file or directory
collect2: error: ld returned 1 exit status

[wantehchang]

Steps to reproduce:

1. cd ext
2. ./avm.cmd
3. cd ..
4. cmake -S . -B build.avm -G Ninja -DCMAKE_BUILD_TYPE=Debug -DAVIF_CODEC_AVM=LOCAL -DAVIF_LIBYUV=LOCAL -DAVIF_BUILD_TESTS=ON -DAVIF_GTEST=LOCAL
5. cd build.avm
6. ninja

This procedure works for AOM, i.e., in step 2 run ./aom.cmd and in step 4 change AVIF_CODEC_AVM to AVIF_CODEC_AOM.

Since the cmake FetchContent method works well for AVM, this bug may not be worth fixing.