MDEV-35850 make HOSTNAME a cmake configure variable

As seen with openwrt and some other distros, the
determination of hostname can sometime need alternate
commmands.

This provides a cmake option HOSTNAME for non-windows machines
for the mariadb-install-db and mariadbd-safe scripts
and the support-files init scripts..

scripts/CMakeLists.txt

@@ -177,7 +177,6 @@ ELSE()
   SET(CHECK_PID "kill -s SIGCONT $PID  > /dev/null 2> /dev/null")
 ENDIF()
 
-SET(HOSTNAME "uname -n")
 SET(MYSQLD_USER "mysql")
 SET(MYSQLD_GROUP "mysql")
 ENDIF(UNIX)