ALLJOYN-2222: C binding SConscript environment cleanup.

Isolate the C binding SCons environment so that C binding specific compiler
parameter (i.e., -I <include paths>) don't get added to AllJoyn core's
parameters.

Other SConscript file clean ups.

Change-Id: Ifc847f5432c15ffc4833f58119c38bcaf420a2f4