Fix build with newer libssh and cmake

[Dirk Hohndel]

On Sun, Jun 28, 2015 at 10:00:51AM -0700, Dirk Hohndel wrote:
> On Sun, Jun 28, 2015 at 01:59:29PM -0300, Tomaz Canabrava wrote:
> > libssh just changed the way it deals with CMakeFiles and I tougth poor dirk
> > had broken the build, tsc.
> 
> Does this break builds with older libssh?

Well, of course it does:

make
-- out of source build from source in /home/hohndel/subsurface
-- Configuring done
CMake Warning (dev) at CMakeLists.txt:480 (add_executable):
Policy CMP0028 is not set: Double colon in target name means ALIAS or
IMPORTED target.  Run "cmake --help-policy CMP0028" for policy details.
Use the cmake_policy command to set the policy and suppress this warning.

Target "TestGpsCoords" links to target "Libssh2::libssh2" but the target
was not found.  Perhaps a find_package() call is missing for an IMPORTED
target, or an ALIAS target is missing?
Call Stack (most recent call first): CMakeLists.txt:493 (TEST)
This warning is for project developers.  Use -Wno-dev to suppress it.

and then later

Linking CXX executable subsurface
/usr/bin/ld: cannot find -lLibssh2::libssh2
collect2: error: ld returned 1 exit status
CMakeFiles/subsurface.dir/build.make:288: recipe for target 'subsurface' failed

So maybe this needs a little more work, Tomaz?

/D