Fix demo compile issues due to CMake issues finding threading library

[voutilad]

The demos weren't using the CMake FindThreads module to properly identify the threading library for use, which caused both CMake warnings like:

CMake Warning (dev) at CMakeLists.txt:19 (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 "server" links to target "Threads::Threads" but the target was not           
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or         
  an ALIAS target is missing?                                                         
This warning is for project developers.  Use -Wno-dev to suppress it.

And resulted in broken builds where (at least on pthread-based platforms), bogus library args were being passed to the C compiler (-lThreads::Threads):

FAILED: server                                                                        
: && /usr/bin/cc   -rdynamic CMakeFiles/server.dir/server.c.o  -o server  /usr/local/lib/libnng.a -lThreads::Threads -lrt -lpthread -lnsl -latomic && :                      
/usr/bin/ld: cannot find -lThreads::Threads

This updates each demo to include the FindThreads to resolve both the warning and the broken builds.

Note: While in here, I also bumped the CMake version required to match the nng library itself.

[voutilad]

Tested manually on Windows 10 and, assuming the nng library is available for CMake to find, the addition of include(FindThreads) doesn't cause an issue.

[gdamore]

This doesn't feel right. I would have expected that the required threads library would have been pulled in via the nng library. What version of cmake were you using?

[gdamore]

Maybe we need to have the include for FindThreads in the code that is imported. Folks using this library shouldn't have to worry about any of this.

[gdamore]

I think we should probably just add FindThreads to the nng-config.cmake.in file.

[voutilad]

Should the demos be building when someone builds the library? Or are you saying the demos should be able to build relying on the local library source?

[gdamore]

Demos should be able to be built as if they were out of tree -- they represent an example for consumers.

I should probably work up a CI/CD recipe to ensure that demos are buildable at least...

[gdamore]

I can probably fix this properly if you prefer -- I've already looked at what is needed. But if you want to work the issue I'm happy to leave it in your hands as well.

[voutilad]

I’ll take a stab at it. Need to brush up on CMake.

[voutilad]

These haven't been populating properly for quite some time. I'm not sure if it's useful having these properties or not.

[gdamore]

LOl. :-)

[voutilad]

@gdamore this approach seems to be working for me in my limited testing, at least making it so the demos build without modification. Question though: what optional external dependencies should be potentially included in the NNG_PKGS list? Threads is obvious and I've added zerotiercore, but should mbedTLS be in there as well if that's enabled?