build: fix qt5.7 build under macOS (#1469)

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

@@ -38,7 +38,6 @@ else
   CXXFLAGS_overridden=no
 fi
 AC_PROG_CXX
-m4_ifdef([AC_PROG_OBJCXX],[AC_PROG_OBJCXX])
 
 dnl By default, libtool for mingw refuses to link static libs into a dll for
 dnl fear of mixing pic/non-pic objects, and import/export complications. Since
@@ -50,6 +49,16 @@ case $host in
 esac
 dnl Require C++11 compiler (no GNU extensions)
 AX_CXX_COMPILE_STDCXX([11], [noext], [mandatory], [nodefault])
+
+dnl Unless the user specified OBJCXX, force it to be the same as CXX. This ensures
+dnl that we get the same -std flags for both.
+m4_ifdef([AC_PROG_OBJCXX],[
+if test "x${OBJCXX+set}" = "x"; then
+  OBJCXX="${CXX}"
+fi
+AC_PROG_OBJCXX
+])
+
 dnl Libtool init checks.
 LT_INIT([pic-only])