Test for more avx512 functions

Some compiler installations (notably xcode for MacOS El Capitan)
are missing some of the avx512f intrinsics.  As these were not
specifically checked for in configure, it enabled avx512 but
builds failed due to the missing symbols.  Fix by adding some
extra lines to the avx512f configure test so it can turn avx512
off on these platforms.

Author: daviesrob

configure.ac

@@ -188,7 +188,9 @@ HTS_CHECK_COMPILE_FLAGS_NEEDED([avx512f], [-mavx512f -mpopcnt], [AC_LANG_PROGRAM
 	  #ifdef __x86_64__
 	  __m512i a = _mm512_set1_epi32(1);
 	  __m512i b = _mm512_add_epi32(a, a);
-	  return _mm_popcnt_u32(*((char *) &b));
+          __m256i c = _mm512_castsi512_si256(b);
+	  __m256i d = _mm512_extracti64x4_epi64(a, 1);
+	  return _mm_popcnt_u32(*((char *) &c)) + (*(char *) &d);
 	  #endif
 	]])], [
         MAVX512="$flags_needed"