allow tool to specify default timeout

[Janis Johnson]

Daniel Jacobowitz suggested back in April that DejaGnu apply a Debian
patch to use the environment variable DEJAGNU_TIMEOUT to override the
default timeout value:

  http://lists.gnu.org/archive/html/dejagnu/2008-04/msg00001.html

The GCC testsuite now has support for using timeout values from test
directives dg-timeout or dg-timeout-factor or defined in the target
board.  In order to handle this it overrides standard_wait to pass a
timeout to DejaGnu's version of standard_wait.

Please consider support in DejaGnu to override the default by a global
variable defined by the tool, or perhaps by an environment variable so
GCC won't have to override standard_wait and the functionality will be
available to other tools that use DejaGnu.

Janis Johnson