Use mdoc(7) hyperlink macros.
stderr is more appropriately marked up with Dv.
links point to github

i should eventually revive codex.sigpipe.cz,
but this will do for now.
local basename() was shadowed by basename(3)

renamed to tailname()

  logdemux.cpp:183:1: warning: ‘std::string {anonymous}::basename(const string&)’ defined but not used [-Wunused-function]
   basename(string const &path) // {{{
   ^
GNUMakefile: $(link_mode) setting easier to grok

also removed an unnecessary assignment of $(dot_exe).
GNUmakefile: variable assignments better aligned

... and not as cramped
GNUmakefile: generalized through $(name)
GNUmakefile: make sure all is built before installing

prevents possible foot-shootings from running 'all' or 'check'
as root; the root-owned files in the working copy are the least
of problems that could lead to.  the install target depends on
a sentry file which is touched when 'all' succeeds.