Remove CMAKE_LEGACY_CYGWIN_WIN32 which is not needed anymore

According to this from the mailing list, it is no longer required since the minimum cmake required is 3.0.

https://cmake.org/pipermail/cmake/2014-July/058048.html
"The code that emits the warning is run by "project()". Since you do not have an explicit project() call in your top-level CMakeLists.txt CMake adds one to the top implicitly. [1]
A project file that explicitly calls project() after requiring CMake >= 2.8.4 should make the warning go away."

Signed-off-by: Pierre Hallot <hallotpierre@gmail.com>

CMakeLists.txt

@@ -4,8 +4,6 @@
 # To configure the build options either use the CMake gui, or run the command
 # line utility including the "-i" option.
 
-set(CMAKE_LEGACY_CYGWIN_WIN32 0)
-
 project(mosquitto)
 
 cmake_minimum_required(VERSION 3.0)