configure: Enable GCC vectorization on ≥4.9 on x86

4.9 was released precisely nine years after the first GCC version with
autovectorizer (4.0) and six years after the first GCC version with
`-ftree-vectorize` default to enabled on `-O3` (4.3). We've given GCC
enough time to fix those bugs.

FATE passes here on a x86-64 machine with both GCC 4.9.2 and 5.3.1.

Some optimization hotspots benefit greatly from this change, especially
those without handwritten assembly. For instance, the main function in
vf_phase is now 1.6x faster (1.2x overall) on my machine.

configure

@@ -5935,7 +5935,11 @@ elif enabled ccc; then
     add_cflags -msg_disable nonstandcast
     add_cflags -msg_disable unsupieee
 elif enabled gcc; then
-    check_optflags -fno-tree-vectorize
+    case $gcc_basever in
+        4.9*) enabled x86 || check_optflags -fno-tree-vectorize ;;
+        4.*)                 check_optflags -fno-tree-vectorize ;;
+        *)    enabled x86 || check_optflags -fno-tree-vectorize ;;
+    esac
     check_cflags -Werror=format-security
     check_cflags -Werror=implicit-function-declaration
     check_cflags -Werror=missing-prototypes