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.

CMakeLists.txt

@@ -61,6 +61,16 @@ set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_EXTENSIONS ON)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
+# Opt-in ccache launcher. The game sources are frozen (1:1 with the decomp),
+# so ccache makes branch-switch / worktree rebuilds nearly free. No-op when
+# ccache is not installed.
+find_program(CCACHE_PROGRAM ccache)
+if(CCACHE_PROGRAM)
+  set(CMAKE_C_COMPILER_LAUNCHER "${CCACHE_PROGRAM}")
+  set(CMAKE_CXX_COMPILER_LAUNCHER "${CCACHE_PROGRAM}")
+  message(STATUS "ccache enabled: ${CCACHE_PROGRAM}")
+endif()
+
 # Default build type
 if(NOT CMAKE_BUILD_TYPE AND NOT EXISTS ${CMAKE_BINARY_DIR}/CMakeCache.txt)
   set(CMAKE_BUILD_TYPE "RelWithDebInfo" CACHE STRING "" FORCE)

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}"