- disable OpenSWR by default due to broken design (may be enabled only for AVX-capabl...

- disable OpenSWR by default due to broken design (may be enabled only for AVX-capable CPUs); release 2

The issue: not only loadable modules are compiled with -mavx, but also
the common, loader part (because SWR modules API relies on AVX intrinsics).
Even though loader code is not directly executed when swr driver is not
selected, it contains static objects and static initialization code also
may contain AVX instructions, which are executed at the time of loading
the common DSO. It affects libOSMesa, swrast pipe driver and gallium DRI
"megadriver" (there is no separate swr DSO, all drivers are linked into
single DSO, so swr static initialization is executed for any gallium DRI
driver).
(also gallium-xlib libGL is affected, but it's built only in non-DRI case)