Fixed C & C++ compatibility

[monoamine11231]

I noticed the header file was created to allow C & C++ compatibility for this library, but the code itself was much raw C with some exceptions that do not make it possible for it to compile in C.

The following changes were made to make this library compatible with both C++ and C:

• Included the C headers if C and the C++ headers if C++ (<cmath> in C++, but <math.h> in C)
• using namespace std if C++ (no std:: in C)
• typedef struct zbuf zbuf; typedef struct zhuffman zhuffman if C (must be done explicitly in C)
• Proper C-style (same in C++) type casting of values, ex: (int)your_float instead of int(your_float)
• Replaced shadowed struct mixbox_init_t with what it is supposed to do (call decompress)

What's next?

• If accepting my pull request don't forget to update mixbox_lut_compressed with the new code

P.S: When compiling the library in C, you need to link it with math library (-lm option in gcc)
Have a good day.