Upgrade CMake requirement to 3.15

[Alberto Garcia <agarciaillera@xxxxxxxxx>]

Hi,

Right now libssh when built in Windows statically it uses the flags MD and
MDd to set the Run-Time Library (
https://docs.microsoft.com/en-us/cpp/build/reference/md-mt-ld-use-run-time-library?view=vs-2019
).

It makes more sense to use the static version of the RunTime too in case
that we are building libssh statically. To do so, some changes needs to be
made to the libssh CMakefiles. Setting the static Runtime has been
historically a pain and not elegant until CMake 3.15. I have a patch ready
for libssh so the static version gets compile with the static Run-Time but
I'm wondering if you have any problem with raising the CMake minimum
required version from the current 3.3 to the needed 3.15.
Here is a snippet on how it will be used:

cmake_minimum_required(VERSION 3.15)
cmake_policy(SET CMP0091 NEW)
project(my_project)

add_executable(foo foo.c)
set_property(TARGET foo PROPERTY
             MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")



Cheers
-- 
Alberto García Illera

GPG Public Key <https://goo.gl/yshdwh>