This is required for issue852.

The pull request was merged by Florian.

I made a pull request with two modifications: i) the native build options, ii)
ignore an OSI warning that occurred once we removed the "-m64" flag

Link to the pull request:
https://bitbucket.org/gutoblaas/downward/pull-requests/4/issue857

As a side note: the reason we are creating an issue for that, instead of pushing
directly to the master, it is because there is some OSI issues in Windows that
might also be solved in this issue. Hence, we decided it would be better to
document it in a tracked issue.

As mentioned in issue852, we would like to have native build options without the
"-m64" flag. This will be necessary when we start to only support native builds. 

Also, the current 64-bits compilation in Windows generates warnings because of
the "-m64" flags used in the "release64" and "debug64" builds. The first way to
deal with that was to check whether we are using a Windows platform and strip
this flag from the build options, but this was too hacky. The best option now
seems to create the "release-native" and "debug-native" build options and use
them for Windows compilation and the Windows buildbot.