CMake variable for nothreads does not match generated config define

When the mutex manager is set to nothreads, the generated config will not enable the XERCES_USE_MUTEXMGR_NOTHREAD definition.

This is because the configure_file function takes CMake variable names to use for #cmakedefine, but the name for that configuration is XERCES_USE_MUTEXMGR_NOTHREADS, defined in cmake/XercesMutexMgrSelection.cmake.