Windows-GNU: Do not tell find_library to treat '.dll' as linkable

Modern software distributions always use a separate ".dll.a" or ".lib"
import library for linking.

Help/release/dev/mingw-no-find_library-dll.rst

@@ -0,0 +1,8 @@
+mingw-no-find_library-dll
+-------------------------
+
+* When building with GNU tools on Windows (MinGW tools), the
+  :command:`find_library` command will no longer consider
+  ``.dll`` files to be linkable libraries.  All dynamic link
+  libraries are expected to provide separate ``.dll.a`` or
+  ``.lib`` import libraries.

Modules/Platform/Windows-GNU.cmake

@@ -35,7 +35,7 @@ endif()
 
 if(MINGW)
   set(CMAKE_FIND_LIBRARY_PREFIXES "lib" "")
-  set(CMAKE_FIND_LIBRARY_SUFFIXES ".dll" ".dll.a" ".a" ".lib")
+  set(CMAKE_FIND_LIBRARY_SUFFIXES ".dll.a" ".a" ".lib")
   set(CMAKE_C_STANDARD_LIBRARIES_INIT "-lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32")
   set(CMAKE_CXX_STANDARD_LIBRARIES_INIT "${CMAKE_C_STANDARD_LIBRARIES_INIT}")
 endif()