Disable Vc_ENABLE_INSTALL by default

[Vincent-C]

Checklist

• [x] I have described the changes
• [] I have linked to any relevant GitHub issues, if applicable
• [] Documentation in doc/ has been updated
• [] All new code is licensed under GPLv3

Description

Debian maintainer here, forwarded from Debian BTS #1071006

By default, the newly vendored Vc library will be installed alongside conky itself, e.g. all of the headers and the cmake files that ship with Vc. This is unnecessary and even undesired if the user already had Vc installed from other sources, e.g. libvc-dev in Debian/Ubuntu. I can turn this off with -DVc_ENABLE_INSTALL=OFF but I think this would be a sane default option.

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[Caellian]

I believe I should've commented out the installation part of Vc CMake while adding it:

• It's vendored so it might cause other programs to break in the future if we modify functionality and someone overwrites their package installation.
• Conky isn't a library so providing headers is not needed, we just need them at build time.
  • Vc is a static library.

[Caellian]

@Vincent-C let's wait for @brndnmtthws to approve before merging, but I'm pretty sure removing the whole installation part is fine.