marsh: Use the correct feature test macro to use the xercesc
namespace.

XERCES_CPP_NAMESPACE is always defined to be
xercesc_${MAJOR}_${MINOR}, regardless of whether xercesc is
configured/built to use C++ namespaces.  The right feature to check is
XERCES_HAS_CPP_NAMESPACE.
config: Recognize the __ppc macro as signifying PowerPC.

This is how it is represented under the VxWorks 6.9 GNU compiler.
Add VxWorks 6.9 platform.