Add option for omitting custom targets from "first", "default" and "all" targets

In some cases, it is useful to have a subdirectory that is not built by default. At the moment, anything that is added to the SUBDIRS variable is automatically built when "make" is called, but it would be useful if this subdirectory had the option of being excluded from the "first", "default", (and maybe "all") targets so that it could be invoked optionally by typing "make <target name>" at a later time.