I have some problems
building fmtconv for x64 using Mingw64.
Created an issue on your git :
https://github.com/EleonoreMizo/fmtconv/issues/14
After
applying the patch I attached at the issue https://github.com/EleonoreMizo/fmtc...fmtc.patch.txt , the building works ok, but I'm not sure my patch doesn't mess up anything.....