Fix CMAKE_DL_LIBS on Cygwin

The variable should contain the name of a library needed to link the
symbol equivalent to dlopen.  On Cygwin no special library is needed,
and certainly not "gdi32".

Change based on patch from issue #10122.

Modules/Platform/CYGWIN-GNU.cmake

@@ -18,7 +18,6 @@ if(__CYGWIN_COMPILER_GNU)
 endif()
 set(__CYGWIN_COMPILER_GNU 1)
 
-set(CMAKE_DL_LIBS "-lgdi32" )
 set(CMAKE_EXE_LINKER_FLAGS_INIT "-Wl,--enable-auto-import")
 set(CMAKE_CREATE_WIN32_EXE  "-mwindows")