Draft: Various linux-centric development experience fixes

Warning: This is rougher than a bag of bolts and a constant work in progress. The included PKGBUILD is a variant of aur/alchemy-next-viewer-git with some edits to build from the local repository, although it does generate a new source cloning inside the build directory.

scripts/linux_build/linux_native_build.bash

+#!/bin/bash
+do_build() {
+    # TODO: build inside a chroot?
+    virtualenv ".venv" -p python3
+    source ".venv/bin/activate"
+    if command -v autobuild; then
+        abver="$(autobuild --version)"
+        echo "Found ${abver}"
+        if [ "${abver}" = "autobuild 2.1.0" ]; then
+            echo "Reinstalling autobuild to work around some bugs"
+            pip3 uninstall --yes autobuild
+        fi
+    fi
+    pip3 install --upgrade autobuild -i https://git.alchemyviewer.org/api/v4/projects/54/packages/pypi/simple --extra-index-url https://pypi.org/simple
+    # we have a lot of files, relax ulimit to help performance
+    ulimit -n 20000
+    # shellcheck disable=SC2153
+    autobuild configure -A 64 -c ReleaseOS -- -DLL_TESTS:BOOL=OFF -DDISABLE_FATAL_WARNINGS=ON -DUSE_LTO:BOOL="$(grep -cq '[^!]lto' <<< "${OPTIONS}" && echo 'ON' || echo 'OFF')" -DVIEWER_CHANNEL="Alchemy Test" ${AL_EXTRA_CMAKE_FLAGS}
+    cd "build-linux-64" || exit
+    loadavg=$(nproc)
+    if [[ $loadavg -gt 1 ]]; then
+        if [[ $loadavg -le 8 ]]; then loadavg=$((loadavg - 1))
+        else
+            loadavg=$((loadavg - 2))
+        fi
+    fi
+    time ninja -j"${ICECC_PARALLEL_MAKE}" -l"${loadavg}"
+}
+if [[ -z "${IS_PKGBUILD}" ]]
+then
+    if command -v pacman > /dev/null 2>&1; then
+        package_list="dbus-glib glu gtk3 libgl libidn libjpeg-turbo libpng libxss libxml2 mesa nss openal sdl2 vlc zlib cmake gcc python-virtualenv python-pip git boost xz ninja sed"
+        #shellcheck disable=2086
+        if command -v pacman > /dev/null 2>&1; then
+            # work around quoting weirdness
+            echo "$package_list" | xargs pacman -Qq ${package_list} > /dev/null || pkexec pacman --sync --needed ${package_list}
+        fi
+    else
+        eval "$(</etc/lsb-release)"
+        echo "Distribution '${DISTRIB_DESCRIPTION}' is not currently supported by the native build script. Merge requests are welcome."
+        exit 2
+    fi
+fi
+do_build;