fix .3DS error "Chunk is too large" with gcc >=4.7.0 / mingw

[abma]

GCC 4.7.0 breaks attribute((packed)):
http://www.bttr-software.de/forum/mix_entry.php?id=11767

see also the gcc 4.7.0 changelog: http://gcc.gnu.org/gcc-4.7/changes.html

"Windows mingw targets are using the -mms-bitfields option by default."

not sure if my fix is the cleanest approach, but it seems to work... gcc_struct seems to exist since gcc 3.4

backlink to the initial bug report: http://springrts.com/mantis/view.php?id=3972

[acgessler]

Thanks, very good catch! I hope this does not causes issues with older gcc versions though, but we shall see.

[joobei]

I'm getting the same error on OSX with Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn). Built from 3bb8150