Improvements to CMake when LibSSH is not found

[jktjkt]

...by switching from a handcrafted module to pkg-config.

With the original cmake 2.8.11 on CentOS 7.2.1511, a missing LibSSH was
not reported through a usual "required package XYZ not found" message,
but it generated a bit unintuitive error:

  CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
  Please set them or make sure they are set and tested correctly in the CMake files:
  LIBSSH_INCLUDE_DIR
     used as include directory in directory /home/centos/libnetconf2
 (...)
     used as include directory in directory /home/centos/libnetconf2/tests

I suspect that this happens because the custom FindLibSSH.cmake doesn't
set LIBSSH_FOUND to TRUE on success (on my system).

According to rpmfind.net [1], the required pkg-config files have been
shipping since at least the release which is available in EL6's EPEL.

[1] https://www.rpmfind.net/linux/rpm2html/search.php?query=pkgconfig(libssh)

[michalvasko]

Hi,
if you have the correct libssh package installed then the FindLibSSH.cmake module should definitely find it. The error may not be perfect, but it should be obvious what the problem is. The CMake module is written by the libssh team and I don't really see a reason why not to use it.

Regarding skipping -l in the additional linked libraries, are you saying it did not work for you the way it is?

Regards,
Michal

[jktjkt]

if you have the correct libssh package installed then the FindLibSSH.cmake module should definitely find it

Correct; there's no problem if that library is already installed.

The CMake module is written by the libssh team and I don't really see a reason why not to use it.

It doesn't conform to CMake's conventions. A FindFoo.cmake should always set Foo_FOUND to TRUE if that library is available. This bundled file does not set that variable on success -- it only sets it upon failure, which is why the find_package(... REQUIRED) doesn't error out early, and why the cmake process only gets interrupted by trying to use an undefined variable. As you said, a file coming form libssh means that this is a libssh bug. As this case demonstrates, a pkg-config approach would eliminate this error. The decision is yours for sure.

Regarding skipping -l in the additional linked libraries, are you saying it did not work for you the way it is?

Nope, it works already. I changed it because I was investigating another issue, it's just a followup cleanup that I split from my original commit (unpushed) for clarity. Feel free to ignore that one if you don't like it -- I was removing that hardcoded and redundant -lssh_threads so I cleaned up the rest as well.

[michalvasko]

Hi,
it seems to me all this just polishes the build script, but does not fix any actual errors. So thanks for the effort, I will consider all this in the future when I have time for such minor fixes of the whole project, but I will not merge it now.

Regards,
Michal