meson: drop misplaced -Wl,--undefined argument

Ld's man page says the following:

  -u symbol
  --undefined=symbol

  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 5c23128d. AFAICT, this was a mistake. No
such flag appeared in Makefile.am at the time.]

https://github.com/mesonbuild/meson/issues/5113
(cherry picked from commit 700805f6)