Package with x86 vector instruction support

[Jerry James]

I've got a question whose answer I expected to find on one of these
pages, but no joy:

https://fedoraproject.org/wiki/Architectures
https://fedoraproject.org/wiki/Architectures/x86
https://fedoraproject.org/wiki/Architectures/x86-64

I'm packaging a library that does some heavy mathematical
computations.  The build system tries to cleverly determine whether an
Intel CPU with vector instructions is being used.  Until I can
convince upstream to do this at runtime, I'll need to build the
package (a library) for the lowest common denominator.  The package
checks for MMX, SSE, SSE2, SSE3, and SSSE3.  For i686, can I assume
MMX?  SSE?  I believe I can also build an SSE2 version of the library
and drop it into /usr/lib/sse2.  How about for x86_64?  Can I assume
SSE2 is available?  (I thought I could, but I see that /usr/lib64/sse2
exists on my machine, so now I'm uncertain.)  I don't see a
/usr/lib64/sse3 or /usr/lib64/ssse3, so I just have to forget those,
right?

If there is a wiki page that answers these questions, I'd appreciate a
pointer.  If not, perhaps someone with edit privileges could add the
answer to one of the pages listed above.  Thanks,
-- 
Jerry James
http://www.jamezone.org/