Do not add PREFIX to LIBDIR, and make it relative to BUILD

LIBDIR is supposed to be an absolute path, already
including PREFIX. Also fix a few places where 'lib'
was still hardcoded.
Do not add PREFIX to SYSCONFDIR, and make it relative to BUILD

Remove special-cases testing whether SYSCONFDIR is set:
giving it a default value is enough.
Rename BUILD directory from usr/ to build/

With LIBDIR and SYSCONFDIR living below BUILD,
it is no longer equivalent to usr/.
Adapt relative RPATH to LIBDIR

Since now LIBDIR can point to anywhere, and not only to a directory
immediately under PREFIX, it is not guaranteed that $(ORIGIN)/../$(JL_LIBDIR)
points to $(JL_LIBDIR).
Add BINDIR, LIBEXECDIR, DATADIR and INCLUDEDIR

These are needed to get the same directory layout under BUILD as after
installation under DESTDIR and eventually PREFIX.