Allow building on BSD/Solaris/etc with CMake

[alarixnia]

It's not only GNU systems that need libm linked.

Note, the tests still fail for me with: error: ‘M_PIl’ was not declared in this scope; did you mean ‘M_PI’? 46 | long double phinc = 2k0 M_PIl / nfft;

Signed-off-by: Nia Alarie [email protected]

[basilgello]

It's not only GNU systems that need libm linked.

And cross-compilers need -lm, too… Thanks for bringing this out!