The Pi doesn't define 128-bit integer types, and therefore the things that assume [u]int128_t don't work. There is no mode TI on the machine.
Clang does work, but then you should probably state that it's required for the Pi (and whatever else there is that gcc doesn't support the 128-bit types on), or implement 128-bit types with GMP or something (which would be more painful). Also udplib, in the Makefile, assumes clang rather than $(CC); I don't know if that's intentional or not.
udplib : udplib.c udplib.h
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clang udplib.c -o udplib -DTEST=1
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$(CC) udplib.c -o udplib -DTEST=1