configure: Allow linking against a custom libm

Do not link against default libm if one has been supplied via extralibs.
This simplifies building for android armeabi-v7a-hard which requires the custom libm_hard.
strtod: fix building for android armeabi-v7a-hard

    Libc functions always use softfp abi and are marked explicitly in system headers.
    FFmpeg must not override them.