MinGW: Drop find_library support for plain .dll files
The 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.