pip install with skip-build gives errors setup.py:180 on Windows

On trying to pip install dynet after building the solution, I get the following `TypeError message` which causes the install to fail.

```
G:\Code\C++\dynet\build\python>python ..\..\setup.py EIGEN3_INCLUDE_DIR="G:\Code\C++\eigen" build --build-dir="G:\Code\C++\dynet\build" --skip-build install
running build
INFO:root:CMAKE_PATH=F:\Program Files\CMake\bin\cmake.exe
Traceback (most recent call last):
File "..\..\setup.py", line 337, in
py_modules=["dynet", "dynet_viz", "dynet_config"],
File "F:\ProgramData\Anaconda3\lib\distutils\core.py", line 148, in setup
dist.run_commands()
File "F:\ProgramData\Anaconda3\lib\distutils\dist.py", line 955, in run_commands
self.run_command(cmd)
File "F:\ProgramData\Anaconda3\lib\distutils\dist.py", line 974, in run_command
cmd_obj.run()
File "..\..\setup.py", line 180, in run
log.info("MAKE_PATH=" + MAKE_PATH)
TypeError: must be str, not NoneType
```

A possible fix to all of these messages is to use the python string formatting function to create the log messages. I will send a pull request on this.