GDAL_COMPILATION and standalone plugins

When should GDAL_COMPILATION be set ?

I would like a source tree for a driver.
Ideally it would be possible to build this in tree (both
as a plugin or compiled into libgdal.so) and standalone against
an existing gdal installation (with development libraries and headers).

I thought that GDAL_COMPILATION was a way to tell whether my driver
was being built as part of gdal, but I find that
cmake/helpers/SetupStandalonePlugin.cmake includes
cmake/helpers/GdalCompilationFlags.cmake which sets GDAL_COMPILATION

Why do I care ?
An in tree build will declare my GDALRegister function in
    gcore/gdal_frmts.h
(since it will be used in
    frmts/gdalallregister.cpp
) but an out of tree build will need to declare this itself.
I thought that testing GDAL_COMPILATION would be the way to avoid my
GDALRegister function being declared twice under some circumstances.

Any idea what I am missing or misunderstanding ?

Thanks,

-- 
Andrew C. Aitchison                      Kendal, UK
                    [email protected]