BUGFIX: removed duplicate const declarations on a single declaration
statement, now compiles with clang

BUGFIX: changed the datatype of zlevel from uint_8 to int_8, now compiles
with gcc 4.2.1
BUGFIX: json-c 0.11 now uses libjson-c library name, instead of libjson
BUGFIX: it's better to freeze the library dependency: libjson v0.10
merged with nfql/vbajpai