Windows-GNU: Restore find_library treatment of '.dll' as linkable (#15409)

Revert commit v3.3.0-rc1~435^2 (Windows-GNU: Do not tell find_library to
treat '.dll' as linkable, 2015-02-18).  MinGW tools support linking to
'.dll' files directly and many non-CMake build systems still do not
provide a separate '.dll.a' file.

Help/release/3.3.rst

@@ -251,12 +251,6 @@ Deprecated and Removed Features
   Projects may populate the :ref:`User Package Registry` to aid
   users building multiple dependent projects one after another.
 
-* 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.
-
 * The :command:`add_definitions()` command no longer causes a
   :prop_dir:`DEFINITIONS` directory property to be populated. See policy
   :policy:`CMP0059`.

Modules/Platform/Windows-GNU.cmake

@@ -35,7 +35,7 @@ endif()
 
 if(MINGW)
   set(CMAKE_FIND_LIBRARY_PREFIXES "lib" "")
-  set(CMAKE_FIND_LIBRARY_SUFFIXES ".dll.a" ".a" ".lib")
+  set(CMAKE_FIND_LIBRARY_SUFFIXES ".dll" ".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()