fixes for windows and mingw-64

I managed to successfully compile it on Windows 64 using MinGW. I know that the windows wiki was recently updated for MinGW. But not everything is correct there. In [my repo](https://bitbucket.org/WscriChy/ycm-binaries/src/0ba5d148abb655d8bf50e5a030eed78948114631?at=master) there are binaries for Win64 (with libclang.dll included). Also you can recommend my binaries for Vim and Clang on Win64 and/or instructions for building them.

Not everything went smoothly, I still was not able to build examples, mb latter will take a look at this again. You put -Werror flag and have unused variables. I fixed this by removing this flag for MinGW.

Also as the building scripts grow the problem of repeating the same code in both BoostParts and ycm becomes more burdening and error prone. If somebody won't revise the build system I can do this.

Another fixes concerns the python part of the project. In the file `python/extra_conf_store.py ` the path for searching in parent directories was not separated correctly. 

Write me on the email if can help somehow or you did not understand anything in my modifications.