BUG: better check for TIXML_USE_STL
Second attempt after review from !632 (closed).
TIXML_USE_STL to false is completely useless because OTB depends on the STL. Because of !487 (merged), this causes a build error in OTB 7.0.0 when using a tinyxml not from the SuperBuild (because it was set to ON in
- Force TIXML_USE_STL to true in otb_tinyxml.h
- Stop with a fatal error if tinyxml was not built with it