Revert "ast: Fix EXTRA_DIST pal confusion"

This reverts commit 7d544f93799986be90c7d2bc1e03252fc88f779c
since it caused each subdirectory to end up a level too deep
(e.g. all the pal files were in pal/pal/ rather than just pal/).
ast: Rename pal.c to avoid confusion with EXTRA_DIST

See also commit 6d80f16793 and 7d544f937999
ast: Fix ast testers so that they work on Ubuntu

Need to take account of LDFLAGS="-Wl,--no-as-needed"  if set.