Use AC_SEARCH_LIBS() for math functions.

Use it to check whether we need -lm for various math functions -
including floorl().  Let it handle adding -lm, rather than having that
in the various _LDADD macros.

Change-Id: Ic5d24ec35e060306351f4981c92e26879e597d81
Reviewed-on: https://code.wireshark.org/review/5908
Reviewed-by: Guy Harris <guy@alum.mit.edu>