configura.ac: fix test for SSE4.1 assembler support

configura.ac: fix test for SSE4.1 assembler support

This patch modifies the SSE4.1 test 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 SSE4.1 assembly
instructions (when using -O1 and higher), because then the configure test
might incorrectly pass even though the assembler doesn't support the
SSE4.1 instructions (the test will pass because the compiler does support the intrinsics).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91806
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>