[patch] Fix handling of elided libthread-db-search-path values.

[Eli Zaretskii]

> Date: Tue, 10 May 2011 10:37:17 -0700
> From: Doug Evans <dje@google.com>
> Cc: gdb-patches@sourceware.org
> 
> If you omit @var{path}, @samp{libthread-db-search-path} will be reset to
> its default value (an empty list on @sc{gnu}/Linux and Solaris systems).
> Internally, the default value comes from the @code{LIBTHREAD_DB_SEARCH_PATH}
> macro.

OK.