Some support scripts do not ensure that the environment is clean

Support scripts such as support/tag.sh or support/vote.sh do not make sure that no environment influencing the build are set, e.g., it would be possible that C{XX}FLAGS, C[C|XX] or JAVA_HOME might be set in the environment making the produced build not reproducible (there might be more such variables).

The scripts should either reject such a setup or sanitize the environment.