Use GNUInstallDirs in CMake for install flexibility

[eliasdaler]

Right now, we have a lot of folders hard coded for install rules here. I think that for that, we can use GNUInstallDirs module which sets a lot of sensible defaults which can easily be redefined as needed, e.g. if you want libraries to go to some other place than ${CMAKE_INSTALL_PREFIX}/lib, you'll be able to set ${CMAKE_INSTALL_LIBDIR}.

This will make lives of our *BSD folks easier, plus it will allow users of some unconventional distros to be able to configure install directories as they want.

Thoughts?

(Related to #189, #1561, #1510)

[LaurentGomila]

We're happily using that at work to simplify the install configuration. It works fine for our needs on Windows/Linux/MacOS.

[JonnyPtn]

Should be a pretty straightforward change, just need to clarify which variables should be used for what. So I think these are the ones we want to replace, and my suggested replacements. Once the SFML team confirm these a PR can be done pretty quickly

• lib replace with CMAKE_INSTALL_LIBDIR
• SFML_DEPENDENCIES_INSTALL_PREFIX : replace with CMAKE_INSTALL_LIBDIR
• SFML_MISC_INSTALL_PREFIX: replace with
  • CMAKE_INSTALL_INFODIR for the license/readme
  • CMAKE_INSTALL_DOCDIR for the docs
• bin: replace with CMAKE_INSTALL_BINDIR

Not sure what we should do with SFML_PKGCONFIG_INSTALL_PREFIX?

[eliasdaler]

I think for pkg-config files, it'll be good to use ${CMAKE_INSTALL_LIBDIR}/pkgconfig as the default, but still make it configurable (right now, I have lots of *.pc files in /usr/local/lib/pkgconfig and in /usr/lib/...)

[eXpl0it3r]

Don't forget the considerations that were taken into account for the current PKGCONFIG setup in #1433 / #1466 and others. 😉

[eXpl0it3r]

Fixed with #1576