Makefile: Factor out common build arrangements for hacks.

  * Name the `..._SOURCES' variable literally after the hack, so that it
    can be recovered mechanically.

  * Accumulate all of the sources together, into an `ALL_SOURCES' and
    dump that into `DISTFILES', rather than marking them separately for
    distribution.

  * Replace the separate linker rules with a single static-pattern rule.
    This unfortunately needs `.SECONDEXPANSION' to work properly.