Find yaml-cpp: Ignore missing `pkg-config`
Before this update CMake would print a warning message, if `pkg-config`
was not installed.
# ~~~
find_package (PkgConfig QUIET)
+
+if (PKG_CONFIG_FOUND)
pkg_check_modules (PC_YAML-CPP QUIET yaml-cpp)
+endif (PKG_CONFIG_FOUND)
find_path (YAML-CPP_INCLUDE_DIR
NAMES yaml-cpp/yaml.h
