To get _tkinter to work, you have explicitly link -ltk and -ltcl into
_tkinter.so.  At least, on Sun's, this works:

gcc -G _tkinter.o tkappinit.o -L/afs/sipb/project/tcl/arch/sun4x_57/lib -ltcl8.1 -ltk8.1 -o _tkinter.so

Still have to provide LD_LIBRARY_PATH before Tkinter is imported.