Version 1.9.1: Backported patches

This is a patch version that backports fixes from the development of 2.0.

• Support relative inclusion #475
• Fix cases where spaces in paths could break CMake support #471
• Fix an issue with string conversion #421
• Cross-compiling improvement for Conan.io #430
• Fix option group default propagation #450
• Fix for C++20 #459
• Support compiling with RTTI off #461