Provide the option to use the system GDAL install

[tst-ccamp]

Allow users to compile OpenGEE against the system version of GDAL rather than the bundled version of GDAL. This will give users a way to support formats such as MrSID that are not supported by default.

[bradh]

This might avoid the need to solve proprietary library problems for formats JPEG2000 (see #53), MrSID (see #2) and ECW within GEE.

[tst-ccamp]

I've updated the description. We could split this ticket into 2, first ensuring that Fusion always uses the bundled version of GDAL and keep this as a follow-up to enable usage of the system library.

[tst-lsavoie]

If there's no objection, I'd suggest going with the 2 ticket option. We can reset this one to allow using the system GDAL library and open a new one to make sure we're loading the correct library at run time.