BUG: Configure extension AdditionalLauncherSettings only once

In case of SuperBuild extension the settings were
configured in both the top-level and the inner-build directories.

This commit ensures the settings are configured only in the top-level
directory for regular extension and in the inner-build directory for
superbuild ones.

Example of outputs:

• Regular extension:

[...]
-- Setting EXTENSION_NAME ......................: LoadableExtensionTemplate
-- Adding ConfigureAdditionalLauncherSettings target
-- Adding ConfigureAdditionalLauncherSettings target - yes
[...]

• Superbuild extension (top-level configuration):

[...]
-- Setting EXTENSION_NAME ......................: SuperBuildExtensionTemplate
[...]
-- Adding ConfigureAdditionalLauncherSettings target
-- Adding ConfigureAdditionalLauncherSettings target - no (because configuring top-level project)
[...]

• Superbuild extension (inner-build configuration):

[...]
-- Setting EXTENSION_NAME ......................: SuperBuildExtensionTemplate
[...]
-- Adding ConfigureAdditionalLauncherSettings target
-- Adding ConfigureAdditionalLauncherSettings target - yes (because configuring inner-build)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25952 3bd1e089-480b-0410-8dfb-8563597acbee