Switch to GNU99 for clang/gcc

MSVC supports parts of the C99 standard so we don't have to be
that strict anymore:

* C99 _Bool
* C99 compound literals
* C99 designated initializers
* C99 variable declarations

Change-Id: I597e1bb7e359dfb4bcdd32b293d7b76ee4a89355
Reviewed-on: http://review.couchbase.org/40901
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>