Fix "make install" with --enable-relocatable.

When PSPP added support for --enable-relocatable back in 2007, PSPP did
not install any shared libraries.  Now it does, which requires us to
set RELOCATABLE_LIBRARY_PATH in configure.ac.  Otherwise, the installed
binaries will not work automatically but require some manual user
intervention.

Also set RELOCATABLE_STRIP as recommended by the gnulib documentation.