This fixes a build issue with MSVC. While MSVC imports *functions*
from DLLs automatically when building a consumer of the DLL, it does
not import *variables* automatically. In these cases, we need an
explicit __declspec(dllimport).
This commit simply changes our logic to what the libtool manual
suggests, which has a very comprehensive writeup on the topic. Note
that in particular, this solution is carefully designed not to break
static linking. However, as described in the libtool manual,
statically linking the library with MSVC will output warning LNK4217.
This is still the best solution overall, because the warning is
merely a cosmetic issue.
914276e4d2