Last commit caused -xobjective-c to be passed during linking on
Mac, causing object files to be treated as source files. Add a
-xnone flag to AM_LDFLAGS, canceling the effect of -xobjective-c.
Additinally, add a -xobjective-c++ flag to AM_CXXFLAGS, so that we
can use Objective-C in C++ files on Mac, if we ever need to.