David,
I'm a gentoo linux user and it has been noticed that xfractint does not compile with -fno-common or gcc-10 (https://bugs.gentoo.org/706522).
I'm not a C programmer, but I've managed to get the software to compile by applying the three attached patches. Please let me know if you intend to modify your source code so that I know what to do with the corresponding gentoo package.
Well, that is a good question. Where are you getting the source code for XFractint? On the Fractint Developer's website the most recent version should be xtractint-20-04p14.tat.gz. The xfradev.tar.gz should be a pointer to this file. I have been working on an SDL port for a number of years and the original source has not really been maintained. I have been using -std-c99 for my compiling the SDL port using Code::Blocks. I am using KDevelop for XFractint. I haven't compiled that code since 2015. I have upgrade Ubuntu at least once since then and the code does not currently compile. The changes are easy, although I am not sure why they should be necessary. Patch 1 should be a long double, not a double. We would define it as LDBL to accomodate compilers that don't recognize long double. Do those still exist? I'll put your changes in and work on getting XFractint to compile again, but I don't know how long that may take. Jonathan