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/.