ALLJOYN-2223: Set C/C++ include and library paths in build_core.

Make the public C/C++ header file include paths and the library paths default
for all AllJoyn sub-projects.

Change-Id: Ia6f690cf75726a8c8d2913143cc31b298ccb3184
ALLJOYN-2222: SConscript cleanup

Directly look at certain scons command line parameters for creating the base
SCons environment rather than creating the base environment twice.

Change-Id: I46f1c5b3e2c5e3552d2543e67580b9ffb5356d12
ALLJOYN-2224: Smarter build defaults.

Linux platforms use the following defaults:
  OS=linux
  CPU=x86 iff on a 32-bit machine
  CPU=x86_64 iff on a 64-bit machine

Windows platforms use the following defaults:
  OS=win7
  CPU=x86 iff on a 32-bit machine
  CPU=x86_64 iff on a 64-bit machine

MacOSX/iOS uses the following defaults:
  OS=darwin
  CPU=x86 iff on a 32-bit machine
  CPU=x86_64 iff on a 64-bit machine

Change-Id: I51f430c0b08884c3374ac8bb46f7155b85a33392