1. A computer-implemented method performed in a mainframe computer system that comprises a mainframe operating system that is one of a Multiple Virtual Storage (MVS) operating system, a z/OS operating system, or an OS390 operating system, the method comprising: registering one or more supervisor call exit routines with the mainframe operating system; executing a first build file to generate a first build of a software program from a set of files that are stored in a file system; while executing the first build file to generate the first build of the software program, using the one or more supervisor call exit routines to: monitor activity in the file system to determine a plurality of files, from the set of files, that are being accessed in the file system, and record first information about the plurality of files that are being accessed in the file system; wherein monitoring the activity in the file system comprises receiving, from the mainframe operating system, an invocation of at least one of the one or more supervisor call exit routines when a file of the plurality of files is accessed in the file system; based at least on second information that indicates one or more dependencies among the plurality of files, generating a second build file that reflects the one or more dependencies among the plurality of files; wherein the second information is based on the first information; executing the second build file to generate a second build of the software program.