How to fix this error:
Error: Calibre RHEL 5 enabling environment variable is not set to an appropriate value.
// Support for Calibre applications running on the RHEL 5 operating system
// is extremely limited with the Calibre 2019.1-2020.4 releases. Please
// contact Mentor Graphics Customer Support for the environment variable
// setting required to run this version of Calibre on RHEL 5.
// RHEL 5 support is fully discontinued as of the Calibre 2021.1 release.
// For more information, see:
// To disable this message, use "setenv CALIBRE_DISABLE_RHEL5_WARNING 1"