Ld's man page says the following:
Force symbol to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries. -u may be repeated with different option arguments to enter additional undefined symbols. This option is equivalent to the "EXTERN" linker script command.
If this option is being used to force additional modules to be pulled into the link, and if it is an error for the symbol to remain undefined, then the option --require-defined should be used instead.
This would imply that it always requires an argument, which this does not pass. Thus it will grab the next argument on the command line as its argument. Before it took one of the many -lrt args (presumably) and now it grabs something other random linker argument and things break.
[zj: this line was added in the first version of the meson configuration back in 5c23128daba7236a6080383b2a5649033cfef85c. AFAICT, this was a mistake. No such flag appeared in Makefile.am at the time.]
700805f6c5 meson: drop misplaced -Wl,--undefined argument
meson.build | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)