singular: oops, remove again 'shell' Makefile target
ntl: fix install names in libntl
SingularInterface: remove some leftover build files

These hardcoded the PREFIX path
normaliz: build in release mode

This sets NDEBUG, which in turn disables asserts, which in turn avoids
hardcoding the PREFIX path for debug messages. In the future (for
"shipping" versions of the bundle), we may wish to re-enable this again,
as those asserts could be useful. But for now, it helps to have these
gone.