make_release: Tweak 'mingw' building to look for a suitable
wx-config and get the C and C++ compilers to use using its
--cc and --cxx flags.
src/Makefile.am: Ship gdtconvert and gen_img2aven.
src/printwx.cc: Implement a copy ctor for BitValidator in the same
way that wxGenericValidator does.  Older GCC seems to need such a
copy ctor to exist to compile the uses of BitValidator in
printwx.cc.