Only install() PTHREAD_WIN files if it is enabled

[ltjax]

Sorry for the PR bombing, this was all needed to get this to compile with conan!

Previously, PTHREAD_WIN was optional for building, but not for installing. Not it is. I have also unified the way that the variable is read - now: only through CMake, and no longer as an environment variable. Appveyor should already be set up correctly to handle that.

Also, why does Tango handle installation of its dependencies? Shouldn't that be the job of those (or, at least, a different CMake script?)

[bourtemb]

Sorry for the PR bombing, this was all needed to get this to compile with conan!

Previously, PTHREAD_WIN was optional for building, but not for installing. Not it is. I have also unified the way that the variable is read - now: only through CMake, and no longer as an environment variable. Appveyor should already be set up correctly to handle that.

I think it's a valid point. I think we should probably stop using environment variables everywhere and use CMake definitions instead if technically possible.
@NexeyaSGara , do you remember why it was done like that? Or is it simply that we managed to make it work that way at some point and then nobody took the time to improve it? I think there were some issues when not using $ENV but I don't remember what kind of issues.
Do you think we could remove all these $ENV{MY_VAR} in cmake-win.cmake for instance and simply replace them with ${MY_VAR} as done in this PR for PTHREAD_WIN?

Also, why does Tango handle installation of its dependencies? Shouldn't that be the job of those (or, at least, a different CMake script?)

Good question. On Linux, we don't handle the installation of the dependencies, indeed.
I guess this is like that for historical reasons.
The CMake compilation on Windows is relatively recent and the CMakefiles were written with the goal to be able to take advantage of appveyor to compile the library and provide artifacts and to generate some installers so the Tango users would not have to worry about compiling cppTango on Windows any longer. This is now done automagically and the dependencies are provided by the generated installer. Since appveyor seems to be working fine and nobody got the need to compile cppTango on Windows by itself, the files were not touched much after the first successes with the appveyor compilation.
But of course, this could be improved.

[bourtemb]

Looks good to me.
But let's wait for the feedback from @NexeyaSGara who knows this part of the project better before to merge it.

[t-b]

Looks good.

[NexeyaSGara]

Hi,

I did it this way by lack of CMake knowledge. So I'm completly for this kind of improvements.

[NexeyaSGara]

ok