Add code for detecting SSE2 support

Standard cpusupport framework -- the .../Build/ bits detect whether
the compiler supports this and what flags are needed, while the new
cpusupport_x86_sse2.c code checks CPUID.

libcperciva/cpusupport/Build/cpusupport-X86-SSE2.c

@@ -0,0 +1,12 @@
+#include <emmintrin.h>
+
+static char a[16];
+
+int main(void)
+{
+	__m128i x;
+
+	x = _mm_loadu_si128((__m128i *)a);
+	_mm_storeu_si128((__m128i *)a, x);
+	return (a[0]);
+}

libcperciva/cpusupport/Build/cpusupport.sh

@@ -34,4 +34,5 @@ feature() {
 }
 
 feature X86 CPUID ""
+feature X86 SSE2 "" "-msse2" "-msse2 -Wno-cast-align"
 feature X86 AESNI "" "-maes" "-maes -Wno-cast-align" "-maes -Wno-missing-prototypes -Wno-cast-qual"

libcperciva/cpusupport/cpusupport.h

@@ -59,5 +59,6 @@
  * from this list so that the associated C files can be omitted.
  */
 CPUSUPPORT_FEATURE(x86, aesni);
+CPUSUPPORT_FEATURE(x86, sse2);
 
 #endif /* !_CPUSUPPORT_H_ */

libcperciva/cpusupport/cpusupport_x86_sse2.c

@@ -0,0 +1,30 @@
+#include "cpusupport.h"
+
+#ifdef CPUSUPPORT_X86_CPUID
+#include <cpuid.h>
+#endif
+
+#define CPUID_SSE2_BIT (1 << 26)
+
+CPUSUPPORT_FEATURE_DECL(x86, sse2)
+{
+#ifdef CPUSUPPORT_X86_CPUID
+	unsigned int eax, ebx, ecx, edx;
+
+	/* Check if CPUID supports the level we need. */
+	if (!__get_cpuid(0, &eax, &ebx, &ecx, &edx))
+		goto unsupported;
+	if (eax < 1)
+		goto unsupported;
+
+	/* Ask about CPU features. */
+	if (!__get_cpuid(1, &eax, &ebx, &ecx, &edx))
+		goto unsupported;
+
+	/* Return the relevant feature bit. */
+	return (edx & CPUID_SSE2_BIT);
+
+unsupported:
+#endif
+	return (0);
+}