GZDoom Vulkan rendererer issues

[dreamer]

A single line in GZDoom Vulkan renderer uses C++17 feature not available in GCC 5.4 provided by Steam Runtime.

Splitting this issue from #10

I patched the sources to disable building of Vulkan renderer for now. Next step will be to change the GZDoom buildsystem, so I can enable Vulkan renderer again (it should be possible, as I saw Vulkan renderer providing fallback for missing C++17 feature on OpenBSD) and upstream the change, so we can build vanilla GZDoom releases, not our patched version.

To close this issue:

• Fix GZDoom buildsystem to make it possible to disable Vulkan renderer without patching sources
• Enable existing fallback implementation for missing C++17 feature for Steam Runtime builds
• Upstream changes

[dreamer]

Upstream fixed buildsystem issue with enabling/disabling Vulkan renderer before I had a chance to send them patches :)

Version 4.2.0 of GZDoom is now released and packaged. We still need to enable fallback implementation for missing C++17 feature.

[dreamer]

Vulkan renderer is enabled and working now, but it's in "experimental" state and crashes on exit; crash prevents saving configuration file, so you need to disable it manually: (vid_enable_vulkan=0 in ~/.config/gzdoom/gzdoom.ini).