science/openstructure: Add USE_CXXSTD=c++98; Use MASTER_SITES instead of USE_GITLAB

USE_GITLAB doesn't work in this case because it requires GL_COMMIT while only DISTVERSION
should be sufficient.

USE_CXXSTD=c++98 is used because -DCMAKE_CXX_FLAGS='-std=c++98' os overridden.

Reported by: tobik