Build AUR packages with clang and ThinLTO by default

aurb now builds with clang + lld + ThinLTO and bind-mounts a ThinLTO cache and its clang.cfg into the chroot; --gcc keeps the old gcc build. update-kernel still uses gcc.

.local/bin/aurb

@@ -1,15 +1,33 @@
 #!/bin/bash
-# aurb / aurs: aur build / aur sync with ccache mounted in the chroot (use with -c)
+# aurb / aurs: aur build / aur sync with ccache and the ThinLTO cache mounted in the chroot (use with -c).
+# Builds with clang by default (see aurutils-makepkg.conf); --gcc builds with gcc instead.
 case "$(basename "$0")" in
     aurb) cmd=build ;;
     aurs) cmd=sync ;;
     *) echo "Invoke as aurb or aurs" >&2; exit 1 ;;
 esac
 
-CCACHE_HOST_DIR="$HOME/.cache/ccache"
-mkdir -p "$CCACHE_HOST_DIR"
+baseconf="$HOME/.config/aurutils-makepkg.conf"
+args=()
+clang=1
+for arg in "$@"; do
+    if [[ $arg == --gcc ]]; then clang=0; else args+=("$arg"); fi
+done
 
-exec aur "$cmd" \
-    --makepkg-conf "$HOME/.config/aurutils-makepkg.conf" \
+conf=$baseconf
+if ((clang)); then
+    conf=$(mktemp "${XDG_RUNTIME_DIR:-/tmp}/aurb-clang.XXXXXX.conf") || exit 1
+    trap 'rm -f "$conf"' EXIT
+    { echo 'AURB_CLANG=1'; cat "$baseconf"; } > "$conf"
+fi
+
+CCACHE_HOST_DIR="$HOME/.cache/ccache"
+THINLTO_HOST_DIR="$HOME/.cache/thinlto"
+mkdir -p "$CCACHE_HOST_DIR" "$THINLTO_HOST_DIR"
+
+aur "$cmd" \
+    --makepkg-conf "$conf" \
     --bind-rw "$CCACHE_HOST_DIR" \
-    "$@"
+    --bind-rw "$THINLTO_HOST_DIR" \
+    --bind "$HOME/.config/aurb" \
+    "${args[@]}"