Linux, BSD, etc: Install .desktop and .appstream.xml

[lmoureaux]

For desktop environments to find them, the .desktop files need be installed to share/applications. The AppStream XML files go to share/metainfo (and not share/appdata like Freeciv 2.6 does 🤦). This should be done on Linux & friends, let's say any UNIX that is not 🍎.

(share is ${CMAKE_INSTALL_DATAROOTDIR})

[jwrober]

@lmoureaux On a recent Linux compile and install I got these files in my local share directory along with the binaries and other files.

So I think this is fixed/closed but can't seem to find the commit that changed install behavior. Could use someone else to test and see if they get similar behavior.

[lmoureaux]

On a recent Linux compile and install I got these files in my local share directory along with the binaries and other files.

Sorry I missed this comment for too long. Installing the .in files is certainly not correct, the paths inside should be substituted first.