conv_acc: Our code requires SSSE3, not just SSE3

The accelerated convolutional decoder uses SSSE3 instructions such
as PSIGNW (via _mm_sign_epi16) which go beyond what SSE3 offers.  So
let's make sure we use the right compiler flag (-mssse3) and also the
right runtime check.

Without this patch, we would use illegal instructions e.g. on Opteron
Gen3 such as Opteron 2427, which are also used as build.opensuse.org
build hosts (build31 through build36) where we wouldn't pass "make
check" as a result.

Change-Id: I2754164384109f2821fd98ffb48f625893f2923d
Fixes: OS#2386
diff --git a/src/conv_acc_sse_avx.c b/src/conv_acc_sse_avx.c
index 5b6e704..5ac3c16 100644
--- a/src/conv_acc_sse_avx.c
+++ b/src/conv_acc_sse_avx.c
@@ -1,6 +1,6 @@
 /*! \file conv_acc_sse_avx.c
  * Accelerated Viterbi decoder implementation
- * for architectures with both SSE3 and AVX2 support. */
+ * for architectures with both SSSE3 and AVX2 support. */
 /*
  * Copyright (C) 2013, 2014 Thomas Tsou <tom@tsou.cc>
  *