I tend to avoid the word "building" in the context of describing what I do. After all, the compiler does the building :-) "I design and develop software" is what I usually say.

Would be nice if there was a word that captures the idea of someone both designing and constructing the internals of a machine.

I try to avoid the word "design" because most people immediately think of graphic design and get the wrong idea of what I do. Even though it's not as technically accurate, I think "building" or "engineering" are better words to use for people not familiar with software development.