Add configure test to verify SSE2 code compiles.

Make sure that emmintrin.h can be #include'd without causing a
compilation error, rather than blindly defining HAVE_SSE2 based on
architecture.  Attempts to force SSE2 compilation on a 32-bit Ubuntu
13.10 system running as a VMware guest resulted in a no-win choice
without any obvious explanation besides toolchain misconfiguration/bug:
- Suffer compilation failure due to __MMX__, __SSE__, and __SSE2__ not
  being defined, even if -mmmx, -msse, and -msse2 are manually
  specified (note that they appear to be enabled by default).
- Manually define __MMX__, __SSE__, and __SSE2__, and suffer compiler
  warnings that they are already automatically defined.  This results in
  successful compilation and execution, but the noise is intolerable.

configure.ac

@@ -198,21 +198,21 @@ CPU_SPINWAIT=""
 case "${host_cpu}" in
   i[[345]]86)
 	;;
-  i686)
-	JE_COMPILABLE([__asm__], [], [[__asm__ volatile("pause"); return 0;]],
-	              [je_cv_asm])
-	if test "x${je_cv_asm}" = "xyes" ; then
+  i686|x86_64)
+	JE_COMPILABLE([pause instruction], [],
+	              [[__asm__ volatile("pause"); return 0;]],
+	              [je_cv_pause])
+	if test "x${je_cv_pause}" = "xyes" ; then
 	    CPU_SPINWAIT='__asm__ volatile("pause")'
 	fi
+	dnl emmintrin.h fails to compile unless MMX, SSE, and SSE2 are
+	dnl supported.
+	JE_COMPILABLE([SSE2 intrinsics], [
+#include <emmintrin.h>
+], [], [je_cv_sse2])
+	if test "x${je_cv_sse2}" = "xyes" ; then
 	  AC_DEFINE_UNQUOTED([HAVE_SSE2], [ ])
-	;;
-  x86_64)
-	JE_COMPILABLE([__asm__ syntax], [],
-	              [[__asm__ volatile("pause"); return 0;]], [je_cv_asm])
-	if test "x${je_cv_asm}" = "xyes" ; then
-	    CPU_SPINWAIT='__asm__ volatile("pause")'
 	fi
-	AC_DEFINE_UNQUOTED([HAVE_SSE2], [ ])
 	;;
   powerpc)
 	AC_DEFINE_UNQUOTED([HAVE_ALTIVEC], [ ])