Do not find cyg*.dll on Cygwin

While Cygwin supports linking directly to .dll files, the behavior is
now discouraged.  All Cygwin packages now provide import libraries of
the form lib*.dll.a and CMake has built the import libraries for years.

We believe it is now safe to stop explicitly searching for .dll files
because their import libraries will always be available when the
corresponding header files are available.  Users can always set
find_library cache entries to point at a .dll file by hand if they
really must use one.

Change based on patch from issue #10122.

Modules/Platform/CYGWIN.cmake

@@ -11,8 +11,8 @@ SET(CMAKE_EXECUTABLE_SUFFIX ".exe")          # .exe
 # Modules have a different default prefix that shared libs.
 SET(CMAKE_MODULE_EXISTS 1)
 
-SET(CMAKE_FIND_LIBRARY_PREFIXES "cyg" "lib")
-SET(CMAKE_FIND_LIBRARY_SUFFIXES ".dll" ".dll.a" ".a")
+SET(CMAKE_FIND_LIBRARY_PREFIXES "lib")
+SET(CMAKE_FIND_LIBRARY_SUFFIXES ".dll.a" ".a")
 
 # Shared libraries on cygwin can be named with their version number.
 SET(CMAKE_SHARED_LIBRARY_NAME_WITH_VERSION 1)