Customize main source file in Simulink Coder makefile

I am currently generating code for a simulink model using the grt STF and the MinGW64 v4.x Toolchain (in MATLAB 2016a). This setup compiles code that includes the rt_main.c file in the generated makefile. The makefile is not generated from a tmf template but via the coder.make.GmakeWriter. The writer seems to include the rt_main.c because it is part of the BuiltInfo.Src list. It is added to that list by a call to addStandardInfo in ModelBuilder.locNewCodePostCodeGen. This seems to be hardcoded depending on unknown conditions.

My question is now: How can I use the grt STF or make a clone of it, such that my own main.c file is included in the generated makefile with minimal effort but good reusability?