gnulib has its own implementation of fts, which is not affected by the ABI problem, simply because gnulib is statically linked into whichever program you are using it with, so build with transparent LFS as normal.
Since pax doesn't use autotools, I suggest the simplest way to build it would be to build the required libgnu (i.e. a minimal autotools project simply to build libgnu with the fts module), and then patch pax's makefile to link libgnu.a in. This is pretty easy: just take the
configure.ac from a project that uses gnulib (there are many to choose from:
http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=users.txt;h=1b8c87ee4356b5f4d96b44fc654d6985a1efbdc5;hb=HEAD
) and remove everything except for the bits required for gnulib itself.
(You really don't want to try simply copying the required gnulib files into the project as a) that requires a lot of work if you ever want to update to a later version of gnulib code and b) the fts module relies on a lot of other modules, so you might as well let gnulib look after it all for you.)
If you are keen to close this bug and need help with gnulib, I'm happy to help.
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