Fix build with -fno-common: use extern and NO_IMPORT_PYGOBJECT

Define global variables as extern in the header files to fix build with the gcc
option -fno-common.
Also, define NO_IMPORT_PYGOBJECT and NO_IMPORT_PYGTK for PyGTK
as this also fails to build with linker errors if the headers are included multiple
times.

Without those changes, build fails with linker errors like:

  - multiple definition of `\_PyGtk_API
  - multiple definition of `InterfacePrefsType'

when the gcc option -fno-common is used (which we do in the nightly builds against GTK 2.16).
-fno-common is certainly not strictly required but as the changes are not that much, I'd say it's worth.