Add gcc to the AppVeyor environment

appveyor.yml

@@ -1,7 +1,7 @@
 build_script:
 - C:\msys64\usr\bin\bash -lc "pacman --needed --noconfirm -Sy pacman-mirrors"
 - C:\msys64\usr\bin\bash -lc "pacman --noconfirm -Sy"
-- C:\msys64\usr\bin\bash -lc "pacman --noconfirm -S git make"
+- C:\msys64\usr\bin\bash -lc "pacman --noconfirm -S gcc git make"
 test_script:
 - C:\msys64\usr\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER && make -j 32 -k check"
 - C:\msys64\usr\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER && make -j 32 -k check LEGACY=1"