See attached build log.
(Please do not complain about the need for looking at the build log until you can provide an easy way to open bugs with the correct data picked out of a tinderbox log. Thanks.)
Created attachment 248445 [details]
Thanks, fixed. The defect was not in the main TeXmacs build system (which seems well thought-out), but in tiny binaries used in some plugins (I'd say that most of them are unneeded at all - they are used in plugins for very outdated versions of maxima, mathematica, maple). These tiny binaries are produced from tiny C (or C++) files by tiny Makefiles. And they did not respect LDFLAGS (though they respected CFLAGS and CXXFLAGS).