configure: Provide option to explicitly disable AVX2

The configure script detects if the compiler has AVX2 support and
automatically sets avx2_opt="yes" which in turn defines CONFIG_AVX2_OPT.
There is no way of explicitly overriding this setting so this commit adds
two command-line options: --enable-avx2 and --disable-avx2.

The default behaviour, when no option is specified, is to maintain the
current behaviour and enable AVX2 if the compiler supports it.

Signed-off-by: Liam Merwick <Liam.Merwick@oracle.com>
Reviewed-by: Darren Kenny <Darren.Kenny@oracle.com>
Reviewed-by: Mark Kanda <Mark.Kanda@oracle.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configure

@@ -428,7 +428,7 @@ usb_redir=""
 opengl=""
 opengl_dmabuf="no"
 cpuid_h="no"
-avx2_opt="no"
+avx2_opt=""
 zlib="yes"
 capstone=""
 lzo=""
@@ -1329,6 +1329,10 @@ for opt do
   ;;
   --disable-glusterfs) glusterfs="no"
   ;;
+  --disable-avx2) avx2_opt="no"
+  ;;
+  --enable-avx2) avx2_opt="yes"
+  ;;
   --enable-glusterfs) glusterfs="yes"
   ;;
   --disable-virtio-blk-data-plane|--enable-virtio-blk-data-plane)
@@ -1703,6 +1707,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   libxml2         for Parallels image format
   tcmalloc        tcmalloc support
   jemalloc        jemalloc support
+  avx2            AVX2 optimization support
   replication     replication support
   vhost-vsock     virtio sockets device support
   opengl          opengl support
@@ -5032,7 +5037,7 @@ fi
 # There is no point enabling this if cpuid.h is not usable,
 # since we won't be able to select the new routines.
 
-if test $cpuid_h = yes; then
+if test "$cpuid_h" = "yes" -a "$avx2_opt" != "no"; then
   cat > $TMPC << EOF
 #pragma GCC push_options
 #pragma GCC target("avx2")
@@ -5046,6 +5051,8 @@ int main(int argc, char *argv[]) { return bar(argv[0]); }
 EOF
   if compile_object "" ; then
     avx2_opt="yes"
+  else
+    avx2_opt="no"
   fi
 fi