The floating-point build calls a number of math library
functions, and linking with libm is technically necessary.
It wasn't obvious because most systems support shlib
dependencies and pulled it in that way, or supply the
referenced functions with intrinsics. Discovered the issue
trying to build libopusfile against the uninstalled static
libopus, which unlike opus-tools, doesn't itself need libm.

The -lm argument is only added to Libs for the floating-
point (default) build. It's not necessary for the fixed-
point build.

Also mark which build was used in the .pc file description.