pcre: thoroughly clean up object files
Update MKL instructions.
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.