setup.py assumes MSVC on Windows

We'd like to use MinGW on Windows to compile this library, though line 22 is appending "/O2" explicitly:

```python
cargs = ('/O2' if iswindows else '-O3').split()
```

There should probably be something in place to detect if someone is using Microsoft's Compiler or Clang or GCC

Start in setup.py at line 22, where the Windows compiler flags are selected. Check how the build currently identifies the compiler and determine how MinGW, Microsoft's compiler, and Clang should be distinguished. Done means the library can compile on Windows with MinGW without using the MSVC-specific /O2 flag.

Written by the indexing model from the issue text.