configura.ac: fix test for SSE2 & SSSE3 assembler support

configura.ac: fix test for SSE2 & SSSE3 assembler support

This patch modifies the SSE2 & SSSE3 tests in configure.ac to use a
global variable to initialize vector variables. In addition, we now
return the value of the computation instead of 0.

This is done so gcc 4.9 (and lower) won't optimize the SSE assembly
instructions (when using -O1 and higher), because then the configure test
might incorrectly pass even though the assembler doesn't support the
SSE instructions (the test will pass because the compiler does support
the intrinsics).

v2: instead of using volatile, use a global variable as input

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>