config: explictily link in the math library

On some platforms we apparently need to explicitly link in the math library in order to get `log@@GLIBC_...` and `floor@@GLIBC_...`.  See [here](https://groups.google.com/forum/#!topic/ngx-pagespeed-discuss/DmLCG14prKg) and [here](https://github.com/pagespeed/ngx_pagespeed/pull/189#issuecomment-15359680).