Microsoft compiler wrongly assumed on Windows platform

Right now, Bullet 3 is unbuildable using gcc through MinGW on Windows because of Microsoft-specific functions like `sprintf_s` and `fopen_s`. Since MinGW defines the `_WIN32` macro, a better candidate would be checking for `_MSC_VER` to ensure that the Microsoft-specific craziness is only included when MSVC is actually used.

An example from b3OpenCLUtils.cpp
```c++
#ifdef _WIN32
		sprintf_s(binaryFileName,B3_MAX_STRING_LENGTH,"%s/%s.%s.%s.bin",sCachedBinaryPath,strippedName, deviceName,driverVersion );
#else
		sprintf(binaryFileName,"%s/%s.%s.%s.bin",sCachedBinaryPath,strippedName, deviceName,driverVersion );
#endif
```