Allow the user to specify a CMAKE_CXX_STANDARD, keep C++11 default
Allow the user to specify a CMAKE_CXX_STANDARD, keep C++11 default

If the user explicitly sets a `CMAKE_CXX_STANDARD` respect that choice.
Otherwise keep the default C++11 standard.

kimkulling/openddl-parser,kimkulling/openddl-parser,kimkulling/openddl-parser,kimkulling/openddl-parser