Disable SIMD instructions by default.
They can be enabled by setting -DPOCKETFFT_NO_VECTORS=0.

diff --git a/pocketfft_hdronly.h b/pocketfft_hdronly.h
index d75ada6..b2d0a23 100644
--- a/pocketfft_hdronly.h
+++ b/pocketfft_hdronly.h
@@ -39,6 +39,10 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #ifndef POCKETFFT_HDRONLY_H
 #define POCKETFFT_HDRONLY_H
 
+#ifndef POCKETFFT_NO_VECTORS
+#define POCKETFFT_NO_VECTORS 1
+#endif
+
 #ifndef __cplusplus
 #error This file is C++ and requires a C++ compiler.
 #endif
@@ -106,29 +110,29 @@ constexpr bool FORWARD  = true,
                BACKWARD = false;
 
 // only enable vector support for gcc>=5.0 and clang>=5.0
-#ifndef POCKETFFT_NO_VECTORS
-#define POCKETFFT_NO_VECTORS
+#if !(POCKETFFT_NO_VECTORS)
+#define POCKETFFT_NO_VECTORS 1
 #if defined(__INTEL_COMPILER)
 // do nothing. This is necessary because this compiler also sets __GNUC__.
 #elif defined(__clang__)
 // AppleClang has their own version numbering
 #ifdef __apple_build_version__
 #  if (__clang_major__ > 9) || (__clang_major__ == 9 && __clang_minor__ >= 1)
-#     undef POCKETFFT_NO_VECTORS
+#define POCKETFFT_NO_VECTORS 0
 #  endif
 #elif __clang_major__ >= 5
-#  undef POCKETFFT_NO_VECTORS
+#define POCKETFFT_NO_VECTORS 0
 #endif
 #elif defined(__GNUC__)
 #if __GNUC__>=5
-#undef POCKETFFT_NO_VECTORS
+#define POCKETFFT_NO_VECTORS 0
 #endif
 #endif
 #endif
 
 template<typename T> struct VLEN { static constexpr size_t val=1; };
 
-#ifndef POCKETFFT_NO_VECTORS
+#if !(POCKETFFT_NO_VECTORS)
 #if (defined(__AVX512F__))
 template<> struct VLEN<float> { static constexpr size_t val=16; };
 template<> struct VLEN<double> { static constexpr size_t val=8; };
@@ -145,7 +149,7 @@ template<> struct VLEN<double> { static constexpr size_t val=2; };
 template<> struct VLEN<float> { static constexpr size_t val=4; };
 template<> struct VLEN<double> { static constexpr size_t val=2; };
 #else
-#define POCKETFFT_NO_VECTORS
+#define POCKETFFT_NO_VECTORS 1
 #endif
 #endif
 
@@ -180,7 +184,7 @@ template<typename T> class arr
     T *p;
     size_t sz;
 
-#if defined(POCKETFFT_NO_VECTORS)
+#if POCKETFFT_NO_VECTORS
     static T *ralloc(size_t num)
       {
       if (num==0) return nullptr;
@@ -3026,7 +3030,7 @@ class rev_iter
 template<typename T> struct VTYPE {};
 template <typename T> using vtype_t = typename VTYPE<T>::type;
 
-#ifndef POCKETFFT_NO_VECTORS
+#if !(POCKETFFT_NO_VECTORS)
 template<> struct VTYPE<float>
   {
   using type = float __attribute__ ((vector_size (VLEN<float>::val*sizeof(float))));
@@ -3139,7 +3143,7 @@ POCKETFFT_NOINLINE void general_nd(const cndarr<T> &in, ndarr<T> &out,
         auto storage = alloc_tmp<T0>(in.shape(), len, sizeof(T));
         const auto &tin(iax==0? in : out);
         multi_iter<vlen> it(tin, out, axes[iax]);
-#ifndef POCKETFFT_NO_VECTORS
+#if !(POCKETFFT_NO_VECTORS)
         if (vlen>1)
           while (it.remaining()>=vlen)
             {
@@ -3245,7 +3249,7 @@ template<typename T> POCKETFFT_NOINLINE void general_r2c(
     constexpr auto vlen = VLEN<T>::val;
     auto storage = alloc_tmp<T>(in.shape(), len, sizeof(T));
     multi_iter<vlen> it(in, out, axis);
-#ifndef POCKETFFT_NO_VECTORS
+#if !(POCKETFFT_NO_VECTORS)
     if (vlen>1)
       while (it.remaining()>=vlen)
         {
@@ -3300,7 +3304,7 @@ template<typename T> POCKETFFT_NOINLINE void general_c2r(
       constexpr auto vlen = VLEN<T>::val;
       auto storage = alloc_tmp<T>(out.shape(), len, sizeof(T));
       multi_iter<vlen> it(in, out, axis);
-#ifndef POCKETFFT_NO_VECTORS
+#if !(POCKETFFT_NO_VECTORS)
       if (vlen>1)
         while (it.remaining()>=vlen)
           {