Do a better job finding Marble Google Sat files

First try if Google Sat is already installed as a provider (and just use
it if it is). Then use the executable path to make an educated guess where
these files might be found as part of Subsurface.

We now install the necessary directory tree under
$(DESTDIR)/usr/share/subsurface/marbledata

Still far from perfect - but this should work at least on Linux. MacOS
will need a different modifier for the path and Windows I haven't even
thought about, yet.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>