Since 00387b2a38 it is possible to construct a dummy Messages object
which does not translate at all. With the old gettext implementation, a
Messages object without a defined language would have used a language from
an environment variable. Therefore, the duplicate definition of _() is no
longer needed. This gettext removal was really a good idea!
1.) Do not include layout tests on MAC, since the
linker-option which allows multiple definitions is
no longer supported.
2.) Indenting made consistent