cross-compile error on Darwin

When trying to create a cross-toolchain on Darwin targeting Linux/nds32, I get following
error from binutils:
(register unsigned int x)
^
/usr/include/strings.h:87:6: note: previous declaration is here
int      fls(int) __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);
         ^
1 warning and 1 error generated.

Renaming the function to fls_internal make the compile go away.