Adds the files to the list of all files where identifiers of the form "@id@" have to be replaced with their values. (Most such identifiers are created with IC_SUBSTITUTE.) - For .in files which contain source code for binaries, use IC_BINARY_ADD_SOURCE instead.
This is a special command to set the actual file name suffix for the so-called ".in files". (See IC_INFILE_ADD for more information about such files.) If you don't use this command, the standard suffix is ".in"; this might be inappropriate for non-Unix platforms - which aren't yet really supported by itchconfig... For example, the standard suffix wouldn't be usable for software packages which shall also work on DOS operating systems because there no file name may contain more than one dot, so "foo.c.in" wouldn't work; for such a project, you could e.g. use IC_INFILE_SUFFIX_SET([in]), i.e. just leave out the dot, and then use IC_INFILE_ADD([foo.cin]) which only contains a single dot and thus works with DOS. If you use this command, it must be called before any IC_INFILE_ADD or IC_BINARY_ADD_SOURCE command.
suffix may only contain alphanumeric characters and dots. It should be somewhat "characteristic"; e.g. IC_INFILE_SUFFIX_SET([.c]) would be a bad idea because the suffix ".c" is already generally used for C source code files.