PATCH: Add --enable-64bit to build 64-bit version of memcached

[dormando]

This and the previous configure.ac change are both in trunk now. I see
no problem with them, and no one else has complained :)

Trond Norbye wrote:
> The attached patch will try to compile a program with -m64 and verify
> that the pointer size is 8 bytes. If so it will add -m64 to the CFLAGS.
> (If not you will need to remove the option or implement support for your
> own compiler ;-)) GCC and Sun Studio (at least version 12) supports -m64)
> 
> Trond
> 
>