Fix the distdir target to cope with spaces in absolute file names.
* lib/am/distdir.am: Quote all instances of $(distdir) and
$(top_distdir). They could contain white space, coming from
a toplevel package bootstrapped with an older Automake version.
(am__relativize): New macro, taken from gnulib code, written by
Bruno Haible.
(distdir): Use it to compute relative paths to distdir and
top_distdir for enclosed subpackages.
Also, quote $(distuninstallcheck_dir).
* lib/am/texinfos.am: Likewise, quote $(distdir).