Fixed incorrect usage of CMAKE_MODULE_PATH.

CMAKE_MODULE_PATH is a list of paths and in this 'configure_file'
line, only the last of the list is required. The modification ensures
that the 'cmake/config.hpp.cmake' file is used directly.

Without this patch, a cmake-time error would occur if CMAKE_MODULE_PATH
was nonempty before adding this 'CMakeLists.txt' file.
Merge pull request #49 from camio/fix-cmakemodulepath-use

Fixed incorrect usage of CMAKE_MODULE_PATH.