lib/am/distdir.am: strip VPATH srcdir from #line in distributed yacc/lex outputs

* bin/automake.in (%ylwrap_dist_files): New. (lang_yacc_target_hook, lang_lex_target_hook): Record distributed outputs and headers. (handle_dist): Define am__ylwrap_dist_files and YLWRAP-DIST.
* lib/am/distdir.am (distdir-am): Strip the $(srcdir)/ prefix from "#line" directives of those files taken from the build directory, preserving their timestamps.
* t/yacc-lex-vpath-dist-line.sh: New test.
* t/list-of-tests.mk: Add it.
* doc/automake.texi (Yacc and Lex): Document it.
* NEWS: Mention it.