Making the CHUNKSIZE setable via environment variable

With this modification the CHUNKSIZE is setable by QV4_MM_MAX_CHUNK_SIZE
environment variable so the memory usage which is important for embedded
devices can be finetuned.

Change-Id: I3cd75158f2255651edd341873de035c1222e3c92
Reviewed-by: Lars Knoll <lars.knoll@digia.com>