Roll out new version of the project template (2018/12/15)

[claudiofantacci]

With this PR we now have a refreshed, shiny and awesome CMake template for exporting Cpp libraries!

In particular I tried to address all the entries of #33 by

• Migrating Travis to Xenial and Xcode9.4, using addons for Homebrew (see Migrate Travis to test on modern Linux distributions? #29).
• Create a matching alias library with namspace prefix, as reported in Chapter 16.2 of C. Scott's Professional CMake (see Create a matching alias library with namspace prefix #30).
• Add header-only library template (see Add header-only library template #31). This change had quite some implications. I found it useful to have a template just for that, as emerged from Any way to modify this for header only libraries ? #28. This also impied the need for the project to export multiple libraries, not just one. Consequently, I had to add a variable with parent scopes to backprop to the main CMakeLists the name of the targets.
• Polish up ✨ the README 😎 (see Update README.md #35).
• Change PATH (legacy keyword) in DIRECTORY in dox custom target (see Change PATH with DIRECTORY in doxygen CMakeLists #38).
• Last, but not least, I tried to provide guidelines and useful links about the complexity of mantaining a C++ library (see Provide hints and useful links on the complexity of mantaining a C++ library  #34).

Let me know what you think and should you have any suggestion for improvements I'll try to address them asap! 👍

Closes #29, closes #30, closes #31, closes #32, closes #34, closes #35, closes #33, closes #38.

[traversaro]

Some minor comments, but they are not blocking the merge at all.
Thanks a lot @claudiofantacci this is amazing!

[traversaro]

By the way, I noticed that we did not mention how to handle static variables in shared libraries in Windows. Unfortunately those are not handled by CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS. While the blog post that we link in the README mention that, I wonder if we should stress this more, or we can simply ignore this relatively niche use case. See ms-iot/ROSOnWindows#33 and ms-iot/ROSOnWindows#37 for a related discussion.

[claudiofantacci]

By the way, I noticed that we did not mention how to handle static variables in shared libraries in Windows. Unfortunately those are not handled by CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS. While the blog post that we link in the README mention that, I wonder if we should stress this more, or we can simply ignore this relatively niche use case. See ms-iot/ROSOnWindows#33 and ms-iot/ROSOnWindows#37 for a related discussion.

Well I guess we can address this as well in this PR. I was unaware of this "obscure issues" (cit.) under Windows. I'll try to understand the content of the linked material (thanks about that!) and then I'll add one more commit to the PR.

In the meanwhile let's try to close all the ongoing discussions 👍

[claudiofantacci]

By the way, I noticed that we did not mention how to handle static variables in shared libraries in Windows. Unfortunately those are not handled by CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS. While the blog post that we link in the README mention that, I wonder if we should stress this more, or we can simply ignore this relatively niche use case. See ms-iot/ROSOnWindows#33 and ms-iot/ROSOnWindows#37 for a related discussion.

Let me add one more reply about this suggestion.
I think we can give proper room to this particular use case, but it requires to change the library a little bit and probably add some more test in Appveyor.
As a matter of the fact, I would probably add a third library that would highlight this particular use case, but in another PR in the next future. What do you think about it?
We can convert your #36 (comment) into an issue for keeping the record of it 👍

[claudiofantacci]

Ping @drdanz 😁

[claudiofantacci]

I updated the PR following @drdanz suggestions.

I also added a long comment about the FIRST_TARGET option of install_basic_package_files here.
Please have a look at it and, if useful, we can polish and modify to including it as a comment of the option in install_basic_package_files itself.

[drdanz]

LGTM now!

[claudiofantacci]

Great! Let's merge it! 🎉