Starting from version 4, iniparser is designed to be thread-safe, provided you surround it with your own mutex logic. The choice to not add thread safety inside the library has been made to provide more freedom for the developer, especially when dealing with their own custom reading logic e.g. acquiring the mutex, reading entries with iniparser, then releasing the mutex.
We have received countless contributions from distrib people to modify the Makefile into what they think is the "standard", which we had to reject. The default, standard Makefile for Debian bears absolutely no relationship with the one from SuSE or RedHat and there is no possible way to merge them all. A build system is something so specific to each environment that it is completely pointless to try and push anything that claims to be standard. The provided Makefile in this project is purely here to have something to play with quickly.
The dumping functions are based on fprintf, which can turn out to be surprisingly slow on some embedded platforms. You can replace fprintf by a combined use of sprintf and fwrite, or you can use setvbuf() to change buffering parameters to accomodate fprintf(). Something like:
setvbuf(f, NULL, _IOFBF, 0);
See the docs: iniparser is a C library. C++ is quite a different language, despite the promises of compatibility. You will have to modify iniparser quite heavily to make it work with a C++ compiler. Good luck!