Using jemalloc heap profiling with MySQL

I spent too much time figuring this out.

This works for me:

MALLOC_CONF="prof:true,prof_gdump:true,prof_prefix:/path/to/files/jez" \
libexec/mysqld ...

This does not work for me:
MALLOC_CONF="prof:true,prof_gdump:true" \
libexec/mysqld ...