Build systems transform source code into binaries or whatever the needed target is for target environment.
The alternative is live systems where the source code is compiled and interned for immediate use in the running system. Examples include: Lisp machines, Smalltalk, and Forth.
Came here to mention Smalltalk. In things like Smalltalk-80 and Squeak, there was no build system, there are no source code files, there isn't anything but the Smalltalk Development Environment. With something like ENVY/Developer, building involved generating an exported image from the environment.
"Within each project, a set of changes you make to class descriptions is maintained. … Using a browser view of this set of changes, you can find out what you have been doing. Also, you can use the set of changes to create an external file containing descriptions of the modifications you have made to the system so that you can share your work with other users.
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The storage of changes in the Smalltalk-80 system takes two forms: an internal form as a set of changes (actually a set of objects describing changes), and an external form as a file on which your actions are logged while you are working (in the form of executable expressions or expressions that can be filed into a system). … All the information stored in the internal change set is also written onto the changes file."
1984 Smalltalk-80 The Interactive Programming Environment page 461
The alternative is live systems where the source code is compiled and interned for immediate use in the running system. Examples include: Lisp machines, Smalltalk, and Forth.
There mught be other paradigms as well.