linux/site.def: use an if !defined() gaurd for DtLocalesToBuild

linux/site.def: use an if !defined() gaurd for DtLocalesToBuild

Without this, you cannot use the command line to select the languages
to build, you would have to edit site.def directly.

With this fix, you can build a language on linux with something like:

make World IMAKE_DEFINES='-DDtLocalesToBuild="de_DE.ISO8859-1 es_ES.ISO8859-1"'

to build the DE (German) and es (Spanish) locales.