MinGW: Drop find_library support for plain .dll files
CMAKE_FIND_LIBRARY_SUFFIXES variable used by
find_library to consider file suffixes for linkable libraries contains
.dll when using MinGW tools. At least at one time it was possible to link directly to a
.dll instead of using an import library. However, in practice packages tend to always provide the
.dll.a import library, and it is wrong to find a MSVC-ABI
.dll too. Maybe we should drop plain
.dll from the default list of extensions on MinGW.