Fix osx detection, we do not parse result of os.getversion() in premake ...

...(seems it return something strange). Instead use internal os detection in pre make.

I inserted this in premake script:
```lua
print( os.getversion() )
```
And got something like: ```table: 0x100206dc0``` may be os.getversion() return object... but os.is("osx") work very well.