FFmpeg build failure

[laowangsir]

Ubuntu 16.04 + NDK14b , failed to build ffmpeg in config phrase ，cannot find libx264. The error messages contain libx264.a(ratecontrol-10.o):ratecontrol.c:function x264_10_ratecontrol_new: error: undefined reference to 'log2'

It seems compiler cannot locate some symbols in libdl and libm , but if I add them into extra-libs ,it cannot find the library.