build: opt-in ccache + BUILD_DIR override

CMakeLists.txt: CMAKE_{C,CXX}_COMPILER_LAUNCHER=ccache when found (no-op
otherwise); game sources are frozen so branch-switch rebuilds get near-free.
build-pc.sh: BUILD_DIR=${BUILD_DIR:-build-pc} so parallel agents/worktrees
can use separate build dirs.

build-pc.sh

@@ -22,7 +22,7 @@
 set -euo pipefail
 
 ROMID="${1:-ntsc-final}"
-BUILD_DIR="build-pc"
+BUILD_DIR="${BUILD_DIR:-build-pc}"
 
 echo "==> Configuring PC port (ROMID=${ROMID})"
 cmake -S . -B "${BUILD_DIR}" -DROMID="${ROMID}"