Try to build libaom with gcc

appveyor.yml

@@ -1,32 +1,16 @@
 image: Visual Studio 2017
-configuration: Release
 clone_folder: C:\projects\go-avif\src\github.com\Kagami\go-avif
 
 install:
-  # Vars
+  - set PATH=%PATH%;C:\msys64\mingw64\bin
-  - call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"
-  - set PATH=%PATH%;C:\projects;C:\msys64\mingw64\bin
   - set GOPATH=C:\projects\go-avif
-  - set "CGO_CFLAGS=-I../../../../../aom"
+  - set CGO_LDFLAGS=-static
-  - set "CGO_LDFLAGS=-static -L../../../../../aom_build/%CONFIGURATION%"
+  - C:\msys64\usr\bin\pacman --noconfirm -S mingw-w64-x86_64-cmake mingw-w64-x86_64-yasm
-  # Tools
+  - C:\msys64\usr\bin\bash -lc "cd /c/projects && git clone --depth=1 https://aomedia.googlesource.com/aom && mkdir aom_build && cd aom_build && cmake ../aom -G 'MSYS Makefiles' -DCMAKE_BUILD_TYPE=Release -DCONFIG_AV1_DECODER=0 -DENABLE_DOCS=0 -DENABLE_EXAMPLES=0 -DENABLE_TOOLS=0 -DENABLE_TESTS=0 -DCONFIG_LOWBITDEPTH=1 && make -j4 && make install"
-  - cd C:\projects
-  - appveyor DownloadFile http://www.tortall.net/projects/yasm/releases/yasm-1.3.0-win64.exe -FileName yasm.exe
-  # libaom
-  - IF NOT EXIST aom git clone https://aomedia.googlesource.com/aom
-  - cd aom && git pull && cd ..
-  - mkdir -p aom_build && cd aom_build
-  - cmake ../aom -G "Visual Studio 15 2017" -A x64 -DCONFIG_AV1_DECODER=0 -DENABLE_DOCS=0 -DENABLE_EXAMPLES=0 -DENABLE_TOOLS=0 -DENABLE_TESTS=0 -DCONFIG_LOWBITDEPTH=1
-  - cmake --build . --config %CONFIGURATION%
 
 build_script:
-  - cd %APPVEYOR_BUILD_FOLDER%
   - go get -v ./...
 
 artifacts:
   - path: ..\..\..\..\bin\avif.exe
     name: $(APPVEYOR_PROJECT_NAME)
-
-cache:
-  - C:\projects\aom
-  - C:\projects\aom_build