The 4th argument of AC_SEARCH_LIBS is a list of additional libraries,
not the name of the variable to hold the result which is always
ac_cv_search_$function. Also, we should ignore the result when it is
"none required".

Fixes: 1f2eeb85 ('build: rename $(LIBDL) to $(DL_LIBS) and modify detection')
(cherry picked from commit bd4957fc)