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..

CMakeLists.txt

@@ -379,6 +379,7 @@ IF(WIN32)
 ELSE()
   SET(DEFAULT_MYSQL_HOME ${CMAKE_INSTALL_PREFIX})
   SET(SHAREDIR ${INSTALL_MYSQLSHAREDIRABS})
+  SET(HOSTNAME "uname -n" CACHE STRING "Command for determining hostname")
 ENDIF()
 
 SET(DEFAULT_BASEDIR "${DEFAULT_MYSQL_HOME}")