That's handled by the preprocessor. It's literally just a "insert the contents of that file here" copy-paste.
Also, going to this level of trouble to support systems that don't have files seems... odd. Targets that don't have files, that I can totally understand. But compiler toolchains that don't have the notion of a file? That sounds obscure beyond obscure. I'm surprised such a system would be a compilation host instead of a cross-compile target.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n421...
int main(int argc, char *argv<::>)
<%
if (argc not_eq 1 and argc not_eq 2) <% return 1; %>
return 0;
%>
https://en.wikipedia.org/wiki/Digraphs_and_trigraphs#C