even blink at them they seem to break due to being linkonce (if compiled with
any optimization level > 0), so let's give them weak linkage instead.
The difference is that unreferenced linkonce symbols can be deleted, while
weak symbols need to be preserved.
Also moved the #defines for linkage types into a separate header instead of
mars.h so we can #include revisions.h without having to rebuild the entire
frontend every time we update.
(I'm using revisions.h to get the LLVM revision for use in preprocessor
conditionals. It should work with LLVM release 2.5, old trunk and new trunk)
