Index: avr-libc-1.7.1/include/util/delay.h.in
===================================================================
--- avr-libc-1.7.1/include/util/delay.h.in
+++ avr-libc-1.7.1/include/util/delay.h.in	2015-09-06 17:14:49.176289495 +0200
@@ -94,6 +94,12 @@
 # warning "Compiler optimizations disabled; functions from <util/delay.h> won't work as designed"
 #endif
 
+#if __HAS_DELAY_CYCLES && defined(__OPTIMIZE__) && \
+  !defined(__DELAY_BACKWARD_COMPATIBLE__) &&      \
+  __STDC_HOSTED__
+#include <math.h>
+#endif
+
 /**
    \ingroup util_delay