Version 2.1.3

Added:

• Added <libassert/version.hpp> header with version macros
• Added export set definition
• Made the default failure handler callback public, updated docs on custom failure handlers

Fixes:

• Added a flag to control -Werror instead of having it on unconditionally by default
• Added fallback for older cpptrace versions where <cpptrace/basic.hpp> didn't exist
• Added safeguard to ensure static init order issues can never bite

Other:

• Some internal refactoring
• Bumped to cpptrace 0.7.4