build(linux): allow skipping file installation

[embik]

I'm one of the Gentoo chads called out here:

vicinae/CMakeLists.txt

Line 26 in 9878d64

option(LTO "Enable Link Time Optimization (LTO). This will result in better performance, but greatly increased compile time. (Gentoo chads can't live without this)" OFF)

I am admittedly new to writing ebuilds (the packaging instructions for Gentoo), but noticed that the unconditional attempt to install files into CMAKE_INSTALL_PREFIX (/usr in practice, I suppose) was violating the build sandbox for my ebuild.

Instead of letting cmake install those files (themes, .desktop files, icons, systemd unit), I would like the package manager to do it from extra/ directly.

This adds INSTALL_SHARED_FILES, ON by default (so my little snowflake distribution doesn't break everyone else's build/install process), which can be toggled OFF to delegate installation of files to an external process. Hope this is useful, if not please feel free to close.

[arturict]

For linux build changes, a quick note on required environment variables in manifest generation would help review and troubleshooting.