Port Windows plugins to non-CMake builds #743
Description
The core code for loading plugins seems to work but it likely requires a very little bit of configury (see 1b66dac). Building actual plugins without CMake requires more work.
It's not clear whether CMake-built plugins are usable with non-CMake hwloc.dll (are symbols even compatible between MSVC/CMake builds and MinGW or Cygwin?), but I am not sure that would be useful to anybody anyway. So we likely want to support building plugins before enabling loading them.