Account for variable differences
When checking for a version of pybind11, if a "find_package" call is used, the casing of the variables for the version numbers will be different. Account for that difference when creating the PYBIND11_VERSION string.
When checking for a version of pybind11, if a "find_package" call is used, the casing of the variables for the version numbers will be different. Account for that difference when creating the PYBIND11_VERSION string.