COMP: Do not include or use i18n macros if Slicer_BUILD_I18N_SUPPORT is OFF

Commit r21856 disabled internationalization support by default and
it introduced the option Slicer_BUILD_I18N_SUPPORT allowing to enable it.

This commit takes care of excluding the associated CMake macro and its
invocation if the option is disabled.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21865 3bd1e089-480b-0410-8dfb-8563597acbee