Certain variables have to be exported in order for the system tests to
work. It makes little sense to export the variables in one place/script
while they're defined in another place.
Since it makes no harm, export all the variables to make the behaviour
more predictable and consistent. Previously, some variables were
exported as environment variables, while others were just shell
variables which could be used once the configuration was sourced from
another script. However, they wouldn't be exposed to spawned processes.
For simplicity sake (and for the upcoming effort to run system tests
with pytest), export all variables that are used. TESTS, PARALLEL_UNIX
and SUBDIRS variables are automake-specific, aren't used anywhere else
and thus not exported.
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