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.

spice-gtk.spec

@@ -117,8 +117,9 @@ find . -name '*.stamp' | xargs touch
     --enable-webdav \
     --with-usb-acl-helper-dir=%{_libexecdir}/spice-gtk-%{_arch}/ \
     --enable-lz4 \
-    --enable-vala
-make %{?_smp_mflags}
+    --enable-vala \
+    --disable-werror
+make %{?_smp_mflags} V=1
 
 
 %install