Handle present but cannot be compiled warning

Need to expand the line:

AC_CHECK_HEADERS([sys/time.h c_asm.h intrinsics.h mach/mach_time.h])

to follow the pattern:

AC_CHECK_HEADERS([immintrin.h], [], [],
[#ifdef HAVE_IMMINTRIN_H
# include <immintrin.h>
#endif
])

So that we handle the case when the C preprocessor is happy with the headers, but they do not compile without generating lots of warnings.