Make HAVE_ATTRIBUTE_TARGET check also check SSSE3 intrinsics work

GCC 4.8.5 and earlier accept SSSE3 intrinsics with -mssse3; these
compiler versions also accept __attribute__((target("ssse3"))) but
the attribute fails to enable compilation of the intrinsics.

Refactor the check to HAVE_ATTRIBUTE_TARGET_SSSE3, and check both
the attribute and that the SSSE3 intrinsics used (in simd.c) are
in fact compilable in a function with the attribute. Fixes #1838.

Makefile

@@ -309,7 +309,7 @@ config.h:
 	echo '#define HAVE_ATTRIBUTE_CONSTRUCTOR 1' >> $@
 	echo '#endif' >> $@
 	echo '#if (defined(__x86_64__) || defined(_M_X64))' >> $@
-	echo '#define HAVE_ATTRIBUTE_TARGET 1' >> $@
+	echo '#define HAVE_ATTRIBUTE_TARGET_SSSE3 1' >> $@
 	echo '#define HAVE_BUILTIN_CPU_SUPPORT_SSSE3 1' >> $@
 	echo '#endif' >> $@
 	echo '#if defined __linux__' >> $@

configure.ac

@@ -174,16 +174,25 @@ AC_LINK_IFELSE([AC_LANG_PROGRAM([],[
 ])
 
 dnl Check for function attribute used in conjunction with __builtin_cpu_supports
-AC_MSG_CHECKING([for __attribute__((target))])
+dnl and that it does enable the corresponding intrinsics (which is broken on ancient GCCs)
+AC_MSG_CHECKING([for working __attribute__((target("ssse3")))])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+  #ifdef __x86_64__
+  #include "x86intrin.h"
+
   __attribute__((target("ssse3")))
-  int zero(void) {
-    return 0;
+  void shuffle(char *aptr, char *bptr) {
+    __m128i a = _mm_lddqu_si128((__m128i *)aptr);
+    __m128i b = _mm_shuffle_epi8(a, a);
+    _mm_storeu_si128((__m128i *)bptr, b);
   }
-]], [[zero();]])], [
+  #else
+  void shuffle(char *aptr, char *bptr) { }
+  #endif
+]], [[shuffle(0, 0);]])], [
   AC_MSG_RESULT([yes])
-  AC_DEFINE([HAVE_ATTRIBUTE_TARGET], 1,
-            [Define if __attribute__((target(...))) is available.])
+  AC_DEFINE([HAVE_ATTRIBUTE_TARGET_SSSE3], 1,
+            [Define if __attribute__((target("ssse3"))) works.])
 ], [
   AC_MSG_RESULT([no])
 ])

sam_internal.h

@@ -100,7 +100,7 @@ static inline void nibble2base_default(uint8_t *nib, char *seq, int len) {
 }
 
 #if defined HAVE_ATTRIBUTE_CONSTRUCTOR && \
-    ((defined __x86_64__ && defined HAVE_ATTRIBUTE_TARGET && defined HAVE_BUILTIN_CPU_SUPPORT_SSSE3) || \
+    ((defined __x86_64__ && defined HAVE_ATTRIBUTE_TARGET_SSSE3 && defined HAVE_BUILTIN_CPU_SUPPORT_SSSE3) || \
      (defined __ARM_NEON))
 #define BUILDING_SIMD_NIBBLE2BASE
 #endif