[patch] Define __CORRECT_ISO_CPP_STRING_H_PROTO correctly for Clang

[Brooks Moses <bmoses at google dot com>]

In the string/string.h and string/strings.h headers, we have a couple
of macros that "tell the caller that we provide correct C++
prototypes" according to the comment; they are used to determine
whether to wrap some prototypes in "extern "C++"" (and provide
multiple overloads of them, and some other magic) when __cplusplus is
defined.

The macros are set to check for sufficiently-recent GCC versions (4.4
and later), but this is not the right check for non-GCC compilers.  In
particular, these macros should also be set when using Clang -- if
they are not set, then Clang will be unable to correctly diagnose a
number of subtle bugs that will be errors in GCC compilations.

The attached patch thus adds the trivial change to make these headers
work correctly with Clang.  We've been carrying it locally for a
while, but I'd rather have it upstream.

Ok for trunk?

Thanks,
- Brooks

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