Make IPLUG2_DIR a CACHE PATH so it can be overridden

Promoting IPLUG2_DIR to a
CACHE PATH lets a caller override it via -DIPLUG2_DIR=/path on the cmake
command line; the default remains the in-tree submodule so existing
workflows are unaffected.

Author: olilarkin

CMakeLists.txt

@@ -1,7 +1,11 @@
 cmake_minimum_required(VERSION 3.25)
 project(iPlug2OOS)
 
-set(IPLUG2_DIR ${CMAKE_SOURCE_DIR}/iPlug2)
+# iPlug2 framework location. Default to the in-tree submodule, but allow an
+# external canonical checkout via `-DIPLUG2_DIR=/path` or the IPLUG2_DIR
+# environment variable. iPlug2 Agent sets this to a shared per-machine
+# checkout so individual plugin projects don't each carry ~2 GB of deps.
+set(IPLUG2_DIR "${CMAKE_SOURCE_DIR}/iPlug2" CACHE PATH "Path to the iPlug2 framework root")
 include(${IPLUG2_DIR}/iPlug2.cmake)
 
 find_package(iPlug2 REQUIRED)