Check if instancetype supported in ObjC

* configure.ac: Add check for instancetype.
* src/nsterm.h [!NATIVE_OBJC_INSTANCETYPE]: Define instancetype.

configure.ac

@@ -1980,6 +1980,25 @@ AC_SUBST(ns_self_contained)
 AC_SUBST(NS_OBJ)
 AC_SUBST(NS_OBJC_OBJ)
 
+if test "${HAVE_NS}" = yes; then
+  AC_CACHE_CHECK(
+    [if the Objective C compiler supports instancetype],
+    [emacs_cv_objc_instancetype],
+    [AC_LANG_PUSH([Objective C])
+     AC_COMPILE_IFELSE(
+       [AC_LANG_SOURCE([[@interface Test
+                          + (instancetype)test;
+                          @end]])],
+       emacs_cv_objc_instancetype=yes,
+       emacs_cv_objc_instancetype=no)
+     AC_LANG_POP([Objective C])])
+
+  if test x$emacs_cv_objc_instancetype = xyes ; then
+    AC_DEFINE(NATIVE_OBJC_INSTANCETYPE, 1,
+              [Define if ObjC compiler supports instancetype natively.])
+  fi
+fi
+
 HAVE_W32=no
 W32_OBJ=
 W32_LIBS=

src/nsterm.h

@@ -62,15 +62,6 @@ typedef CGFloat EmacsCGFloat;
 typedef float EmacsCGFloat;
 #endif
 
-/* FIXME: instancetype is a language built-in, but older versions of
-   Clang don't support it, and I don't know if GCC supports it at all.
-   Should this be tested for in ./configure? */
-#if defined (NS_IMPL_GNUSTEP)
-    || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_10
-typedef id instancetype;
-typedef NSUInteger NSWindowStyleMask;
-#endif
-
 /* ==========================================================================
 
    Trace support
@@ -365,6 +356,12 @@ char const * nstrace_fullscreen_type_name (int);
 #endif
 
 
+/* If the compiler doesn't support instancetype, map it to id. */
+#ifndef NATIVE_OBJC_INSTANCETYPE
+typedef id instancetype;
+#endif
+
+
 /* ==========================================================================
 
    NSColor, EmacsColor category.
@@ -1302,6 +1299,11 @@ extern char gnustep_base_version[];  /* version tracking */
 #define NSWindowStyleMaskUtilityWindow     NSUtilityWindowMask
 #define NSAlertStyleCritical               NSCriticalAlertStyle
 #define NSControlSizeRegular               NSRegularControlSize
+
+/* And adds NSWindowStyleMask. */
+#ifdef __OBJC__
+typedef NSUInteger NSWindowStyleMask;
+#endif
 #endif
 
 #endif	/* HAVE_NS */