build: Fix shared lib linking for darwin with lld

libtool gets a false-positive from the warning produced by lld -single_module
because it is already the default and unneeded.

Skip the check unconditionally for Darwin linkers.

configure.ac

@@ -83,6 +83,16 @@ case $host in
      lt_cv_prog_compiler_pic="-DPIC"
      lt_cv_prog_compiler_pic_CXX="-DPIC"
   ;;
+  *darwin*)
+     dnl Because it prints a verbose warning, lld fails the following check
+     dnl for "-Wl,-single_module" from libtool.m4:
+     dnl   # If there is a non-empty error log, and "single_module"
+     dnl   # appears in it, assume the flag caused a linker warning
+     dnl "-single_module" works fine on ld64 and lld, so just bypass the test.
+     dnl Failure to set this to "yes" causes libtool to use a very broken
+     dnl link-line for shared libs.
+     lt_cv_apple_cc_single_mod="yes"
+  ;;
 esac
 
 AC_ARG_WITH([seccomp],