GeanyPy: 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.

Closes #120