How to use 'make' or 'external build system' for C-Script?

PLECS
2

You cannot modify the PLECS Make behavior. You can include external libraries directly as C-Code, which works well for simple situations. See Using header file in C-script - #3 by Marco_Guerreiro for an example. However, for more complex situations I would recommend using the DLL block which will give you the most flexibility for compiling your code and linking against external libraries.