Bug 1261263 - Unsupport clang < 3.4. r=froydnj

In bug 1254861, we unsupported clang < 3.3, picking 3.3 essentially
because that's the smallest version we had on automation. Bug 1254854
changed that, and the smallest version on automation is now 3.5.

Now, the motivation to unsupport an old version of clang again is that
recent versions don't have the problem with __float128 used in libstdc++
headers (bug 654493). In fact, starting with clang 3.4, the hack we have
in place is not necessary.

So let's just drop support for clang 3.3 instead of keeping that hack
around longer.

As mentioned in bug 1254854, Ubuntu 12.04 LTS has clang 3.4 packages.

build/autoconf/toolchain.m4

@@ -226,11 +226,13 @@ if test "$GNU_CXX"; then
         dnl from __clang_major__ and __clang_minor__), but the clang that
         dnl comes with Xcode has a completely different version scheme
         dnl despite exposing the version with the same defines.
-        dnl So instead of a version check, check for one of the C++11
-        dnl features that was added in clang 3.3.
-        AC_TRY_COMPILE([], [#if !__has_feature(cxx_inheriting_constructors)
-                            #error inheriting constructors are not supported
-                            #endif],,AC_MSG_ERROR([Only clang/llvm 3.3 or newer supported]))
+        dnl So instead of a version check, do a feature check. Normally,
+        dnl we'd use __has_feature, but there are unfortunately no C++11
+        dnl differences in clang 3.4. However, it supports the 2013-08-28
+        dnl draft of the ISO WG21 SG10 feature test macro recommendations.
+        AC_TRY_COMPILE([], [#if !__cpp_static_assert
+                            #error ISO WG21 SG10 feature test macros unsupported
+                            #endif],,AC_MSG_ERROR([Only clang/llvm 3.4 or newer supported]))
     fi
 
     AC_CACHE_CHECK([whether 64-bits std::atomic requires -latomic],
@@ -330,9 +332,9 @@ EOF
             AC_MSG_ERROR([Your host toolchain does not support C++0x/C++11 mode properly. Please upgrade your toolchain])
         fi
         if test "$host_compiler" = CLANG; then
-            AC_TRY_COMPILE([], [#if !__has_feature(cxx_inheriting_constructors)
-                                #error inheriting constructors are not supported
-                                #endif],,AC_MSG_ERROR([Only clang/llvm 3.3 or newer supported]))
+            AC_TRY_COMPILE([], [#if !__cpp_static_assert
+                                #error ISO WG21 SG10 feature test macros unsupported
+                                #endif],,AC_MSG_ERROR([Only clang/llvm 3.4 or newer supported]))
         fi
 
         CXXFLAGS="$_SAVE_CXXFLAGS"