Is there a dev kit/lib (written in c or c++) to write docx files? Microsoft has a dev kit, but it's written in C#.
Not a kit, but you can try the Word Automation Class, which would work for Word 2007/2010 as well. Details on this answer.
I don't know the answer directly but if you can't find one, can you just writed a C# managed com object that exposes what you need and then just call CoCreateInstance to access the type library from c or c++?
Given the complexity of the XML schema behind OOXML (the Open Office XML file format behind .docx files), you might want to consider using a tool to help you create an XSLT stylesheet that can generate the required output XML. Then use one of the C based XSLT engines (XT or Xalan-c from memory) to do the transform(s) before zipping them up.
Altova's Stylevision looks to have good support for creating the stylesheets but I haven't used it myself. Might be worth a look.
I believe Microsoft intends you to do one of the following things:
1) Have your tool output / manipulate RTF files, which can then be read in by Word 2007.
2) Programatically invoke Word to create the document itself, using its automation API.
I don't think the file format itself is something they generally expose the API to.