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.

Github-Pull: bitcoin#9169
Rebased-From: 70266e9

configure.ac

@@ -45,7 +45,6 @@ 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
@@ -60,6 +59,15 @@ AX_CXX_COMPILE_STDCXX([11], [noext], [mandatory])
 dnl Check if -latomic is required for <std::atomic>
 CHECK_ATOMIC
 
+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])