Rename symbol OGL to DXX_USE_OGL to show that it is a DXX
symbol, not one inherited from a library. Move it to dxxsconf.h to
shorten the command line.
This is a mostly automated transform, but the changes to SConstruct were
manual.
git grep -lzw OGL -- '*.h' '*.cpp' | xargs -0 sed -i -e 's/\(\s*#\s*if\)def\s*OGL/\1 DXX_USE_OGL/' -e 's/\(\s*#\s*if\)ndef OGL/\1 !DXX_USE_OGL/' -e 's/\(\s*#\s*if !\?\)defined(OGL)/\1DXX_USE_OGL/'
62b58e9890