The runtime.c file needs to be compiled by doing the following

   gcc -c -g -std=c99 runtime.c

This will produce a file named runtime.o. The -g flag is to tell the compiler to produce debug information that you may need to use the gdb (or lldb) debugger.

On a Mac with an M1 (ARM) processor, use the -arch x86_64 flag to compile the runtime:

   gcc -c -g -std=c99 -arch x86_64 runtime.c