Makefiles: don't use -xobjective-c when linking files on Mac

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.

app-tools/Makefile.am

@@ -7,6 +7,8 @@ libappwidgets = $(top_builddir)/app/widgets/libappwidgets.a
 
 if PLATFORM_OSX
 xobjective_c = "-xobjective-c"
+xobjective_cxx = "-xobjective-c++"
+xnone = "-xnone"
 endif
 
 if OS_WIN32
@@ -71,3 +73,9 @@ AM_CPPFLAGS = \
 
 AM_CFLAGS = \
 	$(xobjective_c)
+
+AM_CXXFLAGS = \
+	$(xobjective_cxx)
+
+AM_LDFLAGS = \
+	$(xnone)