Fix compilation under LLVM/clang with --enable-gcc-warnings

- When compiling using clang (2.9 or lower) do not enable
  -Wnormalized=id or -Woverride-init when --enable-gcc-warnings
  or --enable-gcc-warnings-advisory is set as these options
  are unsupported.

configure.in

@@ -894,6 +894,11 @@ if test x$enable_gcc_warnings = xyes || test x$enable_gcc_warnings_advisory = xy
 #error
 #endif])], have_gcc43=yes, have_gcc43=no)
 
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [
+#if !defined(__clang__) || (__clang_major__ > 2) || (__clang_major__ == 2 && __clang_minor__ > 9)
+#error
+#endif])], have_clang29orlower=yes, have_clang29orlower=no)
+
   save_CFLAGS="$CFLAGS"
   CFLAGS="$CFLAGS -Wshorten-64-to-32"
   AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [])], have_shorten64_flag=yes,
@@ -924,11 +929,19 @@ if test x$enable_gcc_warnings = xyes || test x$enable_gcc_warnings_advisory = xy
   if test x$have_gcc42 = xyes ; then 
     # These warnings break gcc 4.0.2 and work on gcc 4.2
     # XXXX020 See if any of these work with earlier versions.
-    CFLAGS="$CFLAGS -Waddress -Wmissing-noreturn -Wnormalized=id -Woverride-init -Wstrict-overflow=1"
+    CFLAGS="$CFLAGS -Waddress -Wmissing-noreturn -Wstrict-overflow=1"
+
     # We used to use -Wstrict-overflow=5, but that breaks us heavily under 4.3.
   fi
 
-  if test x$have_gcc43 = xyes ; then 
+  if test x$have_gcc42 = xyes && test x$have_clang29orlower = xno; then
+    # These warnings break gcc 4.0.2 and clang, but work on gcc 4.2
+    # We only disable these for clang 2.9 and lower, in case they are
+    # supported in later versions.
+    CFLAGS="$CFLAGS -Wnormalized=id -Woverride-init"
+  fi
+
+  if test x$have_gcc43 = xyes ; then
     # These warnings break gcc 4.2 and work on gcc 4.3
     # XXXX020 See if any of these work with earlier versions.
     CFLAGS="$CFLAGS -Wextra -Warray-bounds"