configure: Fix srcpath when you run configure in the top-level dir

Using the absolute path to configure, otherwise you get:

  [user@locahost qt5]$ ./configure -minimal
  + cd qtbase
  + ./qtbase/configure -minimal
  ./configure: line 58: ./qtbase/configure: No such file or directory

Change-Id: Ic7568cc571aaa52c56bcd28fa1b6bde8abb32c8e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>