now if use g++4.9 as compiler flag on OSX, it will produce error message
that
```
CMake Error at CMakeLists.txt:2 (project):
The CMAKE_CXX_COMPILER:
g++-4.9
is not a full path and was not found in the PATH.
Tell CMake where to find the compiler by setting
either the environment variable "CXX" or the CMake cache entry
CMAKE_CXX_COMPILER to the full path to the compiler, or to the
compiler name if it is in the PATH.
```