Allow yaml-cpp to be installed from another project (#1007)
Allow yaml-cpp to be installed from another project (#1007)

Changes YAML_CPP_INSTALL from a cmake_dependent_option to an option.

Fixes #756, #847, and #1011.
graehl/yaml-cpp,jbeder/yaml-cpp,dand-oss/yaml-cpp,dand-oss/yaml-cpp,graehl/yaml-cpp,jbeder/yaml-cpp,jbeder/yaml-cpp,jbeder/yaml-cpp,dand-oss/yaml-cpp,graehl/yaml-cpp,dand-oss/yaml-cpp,jbeder/yaml-cpp,graehl/yaml-cpp,dand-oss/yaml-cpp,graehl/yaml-cpp