C++11 const char * constructor overloads for exceptions

[Benjamin De Kosnik]

> This little patch adds the C++11 const char * constructor overloads
> for exceptions.

Seems like a good idea...

but, pick one:

+#ifdef __GXX_EXPERIMENTAL_CXX0X__
+    explicit logic_error(const char* __arg)
+    : logic_error(string(__arg)) { }
+#endif


or

+ 
+    # const char* ctors.
+    _ZNSt11logic_errorC1EPKc;
+    _ZNSt11logic_errorC2EPKc;

Ie, this should not need to be both exported and inlined.
Might as well inline it, and not export it.

-benjamin