time: rearrange+pack vtm and time_object structs

struct time_object shrinks from 88 to 46 bytes on my 64-bit system.

* configure.in: use -Wno-packed-bitfield-compat for GCC 4.4+
  use __attribute__((packed)) if available
* timev.h: shrink and pack struct vtm
* time.c: pack struct time_object and adjust/introduce helpers
  [ruby-core:60794]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45155 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-02-24

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45156 b2dd03c8-39d4-4d8f-98ff-823fe69b080e