Add autoconf tests for size_t and ssize_t. Sort-of resolves public issue 79;
it would solve the problem if MSVC typically used autoconf. However, it gives
a natural place (config.h) to put the typedef even for MSVC.

R=jsbell


git-svn-id: http://snappy.googlecode.com/svn/trunk@80 03e5f5b5-db94-4691-08a0-1a8bf15f6143