Fix static pkg-config compile flags

[henriquejsza]

Summary

• emit YAML_CPP_STATIC_DEFINE through Cflags.private for static pkg-config metadata
• keep shared-library metadata free of the static definition
• add a CTest regression check for both build modes

Fixes #1345

Testing

• static and shared CMake/Ninja Release builds
• ctest (3/3 in each build)
• install and CMake package consumers in both modes
• install and pkg-config consumers, including --static --cflags