How can I include C files in a universal header file?

I am making what you could call a library and instead of including each external c file in my main.c file, I want to make one header file that includes all of my external c files. When I put #include and compile main.c with gcc -I A/Path/On/My/Computer/library.h -o main.o -c main.c, I get the error "in file included in main.c: external_c_file.c - file not found" for each external c file. How can I do this?