std::function cannot be used with bluepill target

The bluepill target is compiled with -fnostdlib which means the bad_function_call exception is not available.

tensorflow/lite/micro/kernels/elu.cc

@@ -17,7 +17,6 @@ limitations under the License.
 
 #include <algorithm>
 #include <cmath>
-#include <functional>
 #include <limits>
 
 #include "tensorflow/lite/c/common.h"
@@ -41,10 +40,11 @@ struct OpData {
   int8_t table[256];
 };
 
+using TransformFunc = float (*)(float);
+
 template <typename T>
 void PopulateLookupTable(const TfLiteTensor* input, const TfLiteTensor* output,
-                         const std::function<float(float)> transform,
-                         OpData* data) {
+                         const TransformFunc transform, OpData* data) {
   if (sizeof(T) != 1) TF_LITE_FATAL("Lookup table valid only for 8bit");
 
   const float inverse_scale = 1 / output->params.scale;
@@ -88,7 +88,7 @@ TfLiteStatus CalculateOpData(TfLiteContext* context, TfLiteNode* node) {
   // Use LUT to handle quantized elu path.
   if (input->type == kTfLiteInt8) {
     OpData* data = static_cast<OpData*>(node->user_data);
-    auto transform = [](float value) {
+    TransformFunc transform = [](float value) {
       return value < 0.0f ? std::exp(value) - 1.0f : value;
     };
     PopulateLookupTable<int8_t>(input, output, transform, data);