When validating, configure with "--prefix=`pwd`/inst"

This means a validate build can be installed locally.
`pwd`/inst probably won't give a useful value on all platforms (in
particular there are probably some Windows configurations it doesn't
work for), but I don't think it will ever make the build fail.

validate

@@ -49,7 +49,7 @@ else
 fi
 
 sh boot
-./configure $config_args
+./configure "--prefix=`pwd`/inst" $config_args
 
 make Validating=YES -j$threads
 fi # testsuite-only