Detect simd headers in c++ instead of cmake

ConflictSet.cpp

@@ -14,6 +14,16 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+#if !defined(USE_SIMD_FALLBACK) && defined(__has_include)
+#if __has_include("immintrin.h")
+#define HAS_AVX 1
+#include <immintrin.h>
+#elif __has_include("arm_neon.h")
+#define HAS_ARM_NEON 1
+#include <arm_neon.h>
+#endif
+#endif
+
 #include "ConflictSet.h"
 #include "Internal.h"
 #include "LongestCommonPrefix.h"
@@ -34,12 +44,6 @@ limitations under the License.
 #include <type_traits>
 #include <utility>
 
-#ifdef HAS_AVX
-#include <immintrin.h>
-#elif defined(HAS_ARM_NEON)
-#include <arm_neon.h>
-#endif
-
 #ifndef __SANITIZE_THREAD__
 #if defined(__has_feature)
 #if __has_feature(thread_sanitizer)