In general, the variable LN_S is 'ln -s', however, LN_S changes to 'cp
-pR' if configure finds the file system does not support symbolic
links.
I was playing with 'ln' for some other reason where I symbolic linked
it to '/bin/false'. That revealed the failure for
src/backend/Makefile. Greping for 'ln -s' revealed three places it's
used. Attaching the patch to fix the same.