Safer linux distro version checks in Qt installation script.

[namark]

This error was reported on discord

-- EXTERNAL_BUILD_ASSETS: https://athena-public.s3.amazonaws.com
-- GLES_OPTION:
Using the Python interpreter located at: /usr/bin/python3.10
-- The CXX compiler identification is GNU 11.1.0
GCC compiler detected, adding -O3 -fPIC -ggdb flags
-- VIRCADIA_OPTIMIZE: true
Adding CPU architecture flags: -march=native -mtune=native
-- VIRCADIA_CPU_ARCHITECTURE: -march=native -mtune=native
 -O3 -fPIC -ggdb -march=native -mtune=native
['/home/ben/build/vircadia/prebuild.py', '--release-type', 'DEV', '--build-root', '/home/ben/build/vircadia/build']
Using a packaged Qt
Traceback (most recent call last):
  File "/home/ben/build/vircadia/prebuild.py", line 194, in <module>
    main()
  File "/home/ben/build/vircadia/prebuild.py", line 119, in main
    qt = hifi_qt.QtDownloader(args)
  File "/home/ben/build/vircadia/hifi_qt.py", line 145, in __init__
    u_major = int( distro.major_version() )
ValueError: invalid literal for int() with base 10: ''
CMake Error at CMakeLists.txt:159 (message):
  prebuild.py failed with error 1

building on Arch linux. According to the docs here the version getters may return an empty string, and otherwise nothing suggest they are necessarily always integers. This PR defaults the major version variables to 0 when parsing the version string as int fails, and removes the unused minor version variables.

[vircadia-build-notifier]

The following links are available:
build (macOS-10.15, client)

• https://athena-public.s3.eu-west-3.amazonaws.com/builds/PR/vircadia_vircadia/PR1587/Vircadia-Interface-PR1587-3fab282-_a20a87d3.dmg

build (ubuntu-18.04, full)

• https://athena-public.s3.eu-west-3.amazonaws.com/builds/PR/vircadia_vircadia/PR1587/Vircadia-PR1587-3fab282-.tar_a20a87d3.Z
• https://athena-public.s3.eu-west-3.amazonaws.com/builds/PR/vircadia_vircadia/PR1587/Vircadia-PR1587-3fab282-_a20a87d3.sh
• https://athena-public.s3.eu-west-3.amazonaws.com/builds/PR/vircadia_vircadia/PR1587/Vircadia-PR1587-3fab282-.tar_a20a87d3.gz

build (macOS-10.15, full)

• https://athena-public.s3.eu-west-3.amazonaws.com/builds/PR/vircadia_vircadia/PR1587/Vircadia-PR1587-3fab282-_a20a87d3.dmg

build (windows-latest, full)

• https://athena-public.s3.eu-west-3.amazonaws.com/builds/PR/vircadia_vircadia/PR1587/Vircadia-PR1587-3fab282-_a20a87d3.exe

build (self-hosted_debian-11_aarch64, full)

• https://athena-public.s3.eu-west-3.amazonaws.com/builds/PR/vircadia_vircadia/PR1587/Vircadia-PR1587-3fab282-.tar_a20a87d3.gz
• https://athena-public.s3.eu-west-3.amazonaws.com/builds/PR/vircadia_vircadia/PR1587/Vircadia-PR1587-3fab282-.tar_a20a87d3.Z
• https://athena-public.s3.eu-west-3.amazonaws.com/builds/PR/vircadia_vircadia/PR1587/Vircadia-PR1587-3fab282-_a20a87d3.sh

[digisomni]

Awaiting testing from Ry with Arch Linux.

[3d1go]

I have tested and confirmed that this works. A nice clear message is presented letting you know what to do if you run into the error. I've also written up a build guide for Arch/Arch-based distros, based on the original guide, which I will share as it's a bit different on Debian/Ubuntu and other distros.

Before the fix:

After the fix: