First, disable function inlining to ensure every function has the code generated in a separate file. To accomplish this, please add the following to the first line in the function definition of every function:
coder.inline('never');
Now that every function will be generated in a separate file, the functions need to be each added to the library. The library will have have an entry for every function that is marked as an Entry-Point Function. To mark functions as such, please use the 'Add Entry-Point Function' button on the second page of the MATLAB Coder wizard, and generate the code again. Every function will now be generated in a separate file and will be callable through the library.
If you want to use the command line "codegen" instead, see the following example:
% Generate code and build a DLL
% In this example, there are three functions defined in separate files
% "main_function" takes no arguments, "support_1" takes a
% constant and "support_2" takes two constants
% The library will have entry points to all functions included
codegen -config:dll main_function support_1 -args {100} support_2 -args {1,5}
