This might seem pedantic, but there is always a build system. Even a basic C compiler knows how to include header files, find the system libraries to link in, and so forth.
A number of languages have compilers that are smart enough to find and build all your code, if you don't have any dependencies. It's, as others noted, the dependencies that create most of the difficulties.
Unless your code is all in one file, your interpreter has to essentially build your app at run time by examining your import statements, and running rules for resolving them.
A number of languages have compilers that are smart enough to find and build all your code, if you don't have any dependencies. It's, as others noted, the dependencies that create most of the difficulties.