Don't remove build directory when building dependencies (needed when CLEANDATE is set)

Ghost Userrequested to merge
fix-building-for-date into master

When building dependencies nestedly (as required when CLEANDATE is set), the build directory while it is still needed in later steps.

Thus the following build would fail:

export CLEANDATE=2018-10-23
./generic/setup.sh clean-lib-sapl linux x64
./generic/fetch.sh clean-lib-sapl linux x64
./generic/build.sh clean-lib-sapl linux x64

Because the build/clean directory would not contain etc any more while it is used in build.sh.

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