Relax tone when warning about cross-compiler names.

Relax tone when warning about cross-compiler names.

* lib/autoconf/programs.m4 (_AC_TOOL_WARN): Support cross-compiles
with poorly named tools; the issue has been reported too many
times in the last four years to pull support.
* doc/autoconf.texi (Specifying Names, Generic Programs): Update
documentation accordingly.
* THANKS: Update.
Reported by Josef Tran and others, wording suggested by Ralf
Wildenhues.

Signed-off-by: Eric Blake <ebb9@byu.net>