acconfig.h config.h.in added --with-threads (taken from gtk+ configure.in)

* acconfig.h
* config.h.in
* configure.in: added --with-threads (taken from gtk+ configure.in)

* libgimp/Makefile.am: ugly hack to force libgimpi.a to get rebuilt since
libgimp.la compilations screws up the deps. Fixes relinking on make install.

* configure.in
* plug-ins/Makefile.am: other cleanups

* libgimp/gimp.c
* app/plug_in.c: pass INT32 instead of INT8 for gdisp_id since that's the
correct type

* app/preferences_dialog.c
* plug-ins/script-fu/script-fu-server.c
* plug-ins/tileit/tileit.c: warning fixups

app/plug-in/plug-in-params.c

@@ -144,7 +144,7 @@ static ProcArg progress_init_args[] =
   { PDB_STRING,
     "message",
     "Message to use in the progress dialog." },
-  { PDB_INT8,
+  { PDB_INT32,
     "gdisplay",
     "GDisplay to update progressbar in, or -1 for a seperate window" }
 };