meson: never require a libm

meson: never require a libm

libm is not guaranteed to exist. POSIX requires the math functions to
exist, but doesn't require to have it be a standalone library.

On platforms where libm exists as a standalone library, it will always
be found by meson -- it is shipped with libc.

If it is not found, then we can safely assume the linker will make the
math functions available by default.

See https://mesonbuild.com/howtox.html#add-math-library-lm-portably

Fixes building with bin_tests=true on Windows.