build: fix qt5.7 build under macOS

OBJCXX's std flags don't get defined by our cxx macro. Rather than hard-coding
to c++11, just force OBJCXX to be the same as CXX unless the user specified
otherwise.

configure.ac

@@ -63,6 +63,15 @@ fi
 AC_PROG_OBJCXX
 ])
 
+dnl Unless the user specified OBJCXX, force it to be the same as CXX. This ensures
+dnl that we get the same -std flags for both.
+m4_ifdef([AC_PROG_OBJCXX],[
+if test "x${OBJCXX+set}" = "x"; then
+  OBJCXX="${CXX}"
+fi
+AC_PROG_OBJCXX
+])
+
 dnl Libtool init checks.
 LT_INIT([pic-only])