Use -rpath also for GSLLIBS if needed

In case gsl-config returns a string containing `-L/path/to/gsl'.
Use patsubst instead of subst to generate RPATHFLAGS

To avoid a problem when a directory name contains `-L':
`-L/DIR-FOR-LAPACK' e.g. would result in
`-Wl,-rpath=/DIR-FOR-Wl,-rpath=APACK'.