Compatible with DMD, but restricted to Windows and functions only.
`export` functions with bodies get the dllexport attribute and will be
exported if the containing object is pulled in when linking.
Body-less `export` functions get the dllimport attribute and will be
accessed via an import table indirection, set up at runtime by the OS.
This is a temporary solution, the proper fix is a pending DMD PR, after
which LDC will need to be adapted.
