MDEV-16676 Using malloc-lib=jemalloc in MariaDB 10.2 causes non-critical error about missing mysql_config on startup

- This commit is based on patch 84fe9720a4d2483ff67b6a and suggestion of
Jean Weisbuch to use `ldconfig -p`
- Format of ldconfig output:
"libjemalloc.so.1 (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/libjemalloc.so.1"

```
$ ./scripts/mysqld_safe
201013 13:36:50 mysqld_safe Adding '/usr/lib/x86_64-linux-gnu/libjemalloc.so.1' to LD_PRELOAD for mysqld
201013 13:36:50 mysqld_safe Logging to '/home/anel/builds/data-10.5/mysqld_safe_anel.err'.
201013 13:36:50 mysqld_safe Starting mysqld daemon with databases from /home/anel/builds/data-10.5
```
Reviewed by: Daniel Black, Faustin Lammler