SimpleXlsxWriter
C++ library for creating XLSX files for MS Excel 2007 and above.
...Hence, if using this library the structure of the future report should be known enough.
The library is written in C++ with using STL functionality and based on the ZIP library (included), which has a free license:
http://www.codeproject.com/Articles/7530/Zip-Utils-clean-elegant-simple-C-Win32
This library is distributed under the terms of the zlib license:
http://www.zlib.net/zlib_license.html