Undefined reference to atof, strtof and etc. with NDK build

[aquastill]

Hi, I try to integrate the prebuilt libs from this demo app to my own project through Android.mk. All building and linking work fine except I got NDK error of Undefined reference to atof, strtof and etc. from libCaffe2_CPU.a and libprotobuf.a.
My NDK version is r11c, just wonder what is the root cause of this error? Is a particular version of NDK or STL required? Thanks.