dist: avoid $(distdir) removal failure on MSYS/MinGW

dist: avoid $(distdir) removal failure on MSYS/MinGW

This change fixes automake bug#10470.

On MSYS (1.0.17) it is not possible to remove a directory that is
in use, and this, together with timing issues, could cause spurious
failures in the cleanup code of the "distcheck" recipe.  In fact,
it should be noted that assuming a directory can be removed while
it the CWD of a running process is a POSIX violation:

  "If the directory is the root directory or the current working
   directory of any process, it is unspecified whether the function
   succeeds, or whether it shall fail and set errno to [EBUSY]."

* lib/am/distdir.am (am__remove_distdir): If rm fails, sleep some
seconds and retry, to give potential "pending" processes some time
to exit and "release" the directory.
* tests/Makefile.am (XFAIL_TESTS): Remove 'distcheck-pr10470.test'.

Report and suggestions by Peter Rosin and Eric Blake.