Compile error on osx

[bastengao]

When I execute 'make', output this log.

In file included from mono_panner.cpp:20:
./wave.h:258:16: warning: absolute value function 'fabsf' given an argument of type 'double' but has parameter of type 'float' which may cause truncation of value
      [-Wabsolute-value]
        return fabsf(2.0f - 4.0f * modf(t + 0.75f, &integral)) - 1;
               ^
./wave.h:258:16: note: use function 'std::abs' instead
        return fabsf(2.0f - 4.0f * modf(t + 0.75f, &integral)) - 1;
               ^~~~~
               std::abs

[3119369616]

This is not an error in fact. It's just a warning of the informal syntax. Please feel free to modify the code.