win32: provide separate macros for weak decls and definitions

win32: provide separate macros for weak decls and definitions

mingw32 seems to want the declaration to also carry the weak attribute.
Strangely, gcc on Linux absolutely does not want the declaration to be marked
as weak.  This may not be the right fix, but it seems to do the trick.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>