Manually force the use of __decltype in C++03 with Clang 3.4.

<string> uses `decltype` in a way incompatible with `__typeof__`.
This is problematic when compiling <string> with Clang 3.4 because
even though it provides `__decltype` libc++ still used `__typeof__`
because clang 3.4 doesn't provide __is_identifier which libc++
uses to detect __decltype.

This patch manually detects Clang 3.4 and properly configures
for it.