Take a look at libXL. There are versions for C, C++ and other languages, and is portable to Windows, linux, Mac, and iOS. It does not require Excel, but can produce xls, xlsx, and xlsm formats that can be opened as workbooks in Excel.
This a commercial library (it isn't free). You can download a demo version that has limited capability if you want to try it out.
There might be open-source libraries available, but I haven't looked.
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