libpostproc: fix S!=B failure

Implement khem's suggestion, ${S}/configure $(EXTRA_OECONF).

NOTE: with or without autotools-brokensep, the following:
| WARNING: QA Issue: ELF binary 'libpostproc.so.52.0.100'
| has relocations in .text

Signed-off-by: Tim Orling <TicoTimo@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>