libdnssec links unnecessarily against libm

During the debian build of the knot package, dpkg-shlibdeps observes that libdnssec.so.6.0.0 is linked using -lm, despite using no symbols from libm.

looks like libm is included thanks to AC_SEARCH_LIBS([pow] in configure.ac, and never dropped by the linker, despite debian throwing in an -Wl,--as-needed as well. I think pow is needed for libknot and libzscanner, but it is not needed for libdnssec. It's not super important, but it would be good to clean this up.