Separate lib names for debug and release build

[oesi82]

When building and installing the lib on windows the release and debug libs are installed in the same folder with the same name, i.e., libnabo.lib. Thus it is not possible to have debug and release lib at the same time. Commonly, a 'd' is used as a suffix to differ the debug from release build. I would be happy if you add
set(CMAKE_DEBUG_POSTFIX "d")
to the CMakeLists.txt.