Add to the configure script the --with-locked-configs option to ignore
environment variable superseding of Modules configurations defined in
modulecmd.tcl script. Lockable configuration option are extra_siteconfig.
Currently locked options are reported through the locked_configs option
on the config sub-command.
This new configure option replces --enable-extra-siteconfig which was
dedicated to the control over the MODULES_SITECONFIG variable.