Before this fix, some sed version where giving warnings and producing an empty file.
This resulted in non-tested files on OSX. After fixing this issue, there are some
failures due to numerical precision that require a test to be fixed.

See https://github.com/plumed/plumed2/issues/782#issuecomment-1005490847

Debian places libraries inside /usr/lib/<arch>, so bindir and libdir
are not subdirectories of the same directory

Related to #737

Fix typo that I noticed