autotools: AX_SSE detects the compiler SSE capabilities

For each SSE feature supported by the compiler

* add the corresponding -msse* flag
* define HAVE_SSE*

Remove AX_EXT because it decides based on the CPU capabilities of the
machine compiling the binary which may or may not be the one running
them.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: gf-complete detects SSE at runtime

And the gf-complete code has been updated to rely on the
gf_internal_t::sse flag instead of #if conditionals.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1512 from dachary/wip-jerasure-sse

 erasure-code: gf-complete detects SSE at runtime

Reviewed-by: Sage Weil <sage@inktank.com>