recursion: remove _AM_EXTRA_RECURSIVE_TARGETS indirection

recursion: remove _AM_EXTRA_RECURSIVE_TARGETS indirection

Since automake automatically smashes extra whitespace and newlines
in the arguments of the macros it traces, we can remove the extra
indirection with the _AM_EXTRA_RECURSIVE_TARGETS.  This doesn't
truly simplify the existing code, but make it slightly easier to
understand anyway, since leave it with one less indirection.

* automake.in: Trace AM_EXTRA_RECURSIVE_TARGETS directly, rather
than tracing _AM_EXTRA_RECURSIVE_TARGETS.  Remove trailing and
leading empty fields in the extracted arguments.
* m4/extra-recurs.m4 (_AM_EXTRA_RECURSIVE_TARGETS): Remove, no longer
needed.
(AM_EXTRA_RECURSIVE_TARGETS): Adjust by making it a simple no-op.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>