Issue #1226 - Explicitly enable sse2 on x86_64 also.

Although it's enabled by default in gcc (should be, anyway!), we're
being explicit here for 64-bit x86 platforms here also. This matches the
old behavior.

build/autoconf/compiler-opts.m4

@@ -179,7 +179,7 @@ if test "$GNU_CC"; then
     CFLAGS="$CFLAGS -fno-math-errno"
     CXXFLAGS="$CXXFLAGS -fno-exceptions -fno-math-errno"
 
-    if test "$CPU_ARCH" = x86; then
+    if test "$CPU_ARCH" = "x86" -o "$CPU_ARCH" = "x86_64"; then
       CFLAGS="$CFLAGS -msse2 -mfpmath=sse"
       CXXFLAGS="$CXXFLAGS -msse2 -mfpmath=sse"
     fi