Explicitly disable -Werror for builds
    
    The use of %autosetup -S git_am clashes with spice-compile-warnings.m4
    handling of -Werror.
    It automatically enables -Werror for builds from git, which are detected
    through the presence of a .git directory is $srcdir. %autosetup will
    create such a dir, so -Werror will be enabled, which is unwanted and a
    change of behaviour from previous builds.