move namespace logic into a separate file. It is useful for users of ABC that need to use symbols without the entire baggage of including abc_global.h

make sure all of ABC and related libraries are in the same namespace (when compiled with ABC_NAMESPACE) by removing extern "C" from function definitions

The previous location of parentheses led to an attempt to compile
against Linux-only clock APIs when building on 32-bit OS X.

Added switch for netlist type to 'send_aig'. Changed defautl to &-space. Fixed printf -> Abc_Print in some places.