I had a similar warning/error/failure when I was simply trying to make an executable from two different object files (main.o and add.o). I was using the command:
gcc -o exec main.o add.o
But my program is a C++ program. Using the g++ compiler solved my issue:
g++ -o exec main.o add.o
I was always under the impression that gcc could figure these things out on its own. Apparently not. I hope this helps someone else searching for this error.