Add Avisynth video encoding workflow

tools/avisynth_runner/build_linux.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+cc="${CC:-cc}"
+script_dir="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
+output="$script_dir/mbt_avs_runner"
+
+pkg_cflags="$(pkg-config --cflags avisynth 2>/dev/null || true)"
+pkg_libs="$(pkg-config --libs avisynth 2>/dev/null || true)"
+
+if [[ -z "$pkg_cflags$pkg_libs" && -z "${CFLAGS:-}${LDFLAGS:-}" ]]; then
+  echo "Could not find avisynth via pkg-config." >&2
+  echo "Set CFLAGS and LDFLAGS manually, or install AviSynth+ development files." >&2
+  exit 1
+fi
+
+"$cc" ${CFLAGS:-} $pkg_cflags -O2 -Wall -Wextra \
+  -o "$output" "$script_dir/mbt_avs_runner.c" \
+  ${LDFLAGS:-} $pkg_libs
+
+echo "built $output"