Fix distclean on Win32

Fix distclean on Win32

nmake realclean in the Encode sub-dirs didn't work because it needed an
extra ../ on the relative paths in @INC. Make the paths absolute instead.

nmake realclean in DynaLoader didn't work either because it tried to make
DynaLoader.c... three times. Move the loop over @ext out of the loop over
@dirs to fix the repetitions, but don't add DynaLoader.c anyway when just
making a 'clean' target.

Also clean up the XSLoader.pm that gets left behind.