sgi extensions

[Benjamin Kosnik]

>I'm about to commit it as follows
>
>      #ifdef __GNUC__
>      #if __GNUC__ < 3
>        #include <hash_map.h>
>        namespace Sgi { using ::hash_map; }; // inherit globals
>      #else
>        #include <ext/hash_map>
>        #if __GNUC_MINOR__ == 0
>          namespace Sgi = std;               // GCC 3.0
>        #else
>          namespace Sgi = ::__gnu_cxx;       // GCC 3.1 and later
>        #endif
>      #endif
>      #else      // ...  there are other compilers, right?
>        namespace Sgi = std;
>      #endif

FYI there are no other compilers officially supported.

-benjamin