scons: Document the need to manually remove .sconsign.dblite

If .sconsign.dblite exists then scons may not rebuild everything.
So to force scons to rebuilt everything you need to remove that
file.

build.txt

@@ -204,11 +204,11 @@ To build gpsd for your host platform from source, simply call 'scons'
 in a working-directory copy. (Cross-build is described in a later
 section.)
 
-To clean the built files, call 'scons -c' .  To clean scons' cache,
-call 'scons sconsclean'.  Doing both should return your working directory
-to a near pristine state as far as building is concerned.  Some user
-created files may remain, and source code changes will not have been
-reverted..
+To clean the built files, call 'scons -c' . To clean scons' cache, call
+'scons sconsclean'.  Run 'rm -f .sconsign.dblite' to clear the scons
+database.  Doing all three should return your working directory to a
+near pristine state as far as building is concerned.  Some user created
+files may remain, and source code changes will not have been reverted..
 
 You can specify the installation prefix, as for an autotools build, by
 running "scons prefix=<installation_root>". The default value is