Revert "Simplify CMake hwy option"

This reverts commit 4d3b1f3.

Author: ssh4net

.claude/settings.local.json

@@ -113,8 +113,18 @@ endif ()
 option (${PROJ_NAME}_BUILD_TOOLS "Build the command-line tools" ON)
 option (${PROJ_NAME}_BUILD_TESTS "Build the unit tests" ON)
 
-# Google Highway SIMD acceleration for selected ImageBufAlgo ops. (optimization)
-option (OIIO_USE_HWY "Enable experimental Google Highway SIMD optimizations (if Highway is available)" OFF)
+# Google Highway SIMD acceleration for selected ImageBufAlgo ops. This is an
+# optional optimization dependency: if enabled but not found, it will be
+# compiled out.
+#
+# Back-compat: honor -DUSE_HWY=OFF by mapping it to OIIO_USE_HWY if the latter
+# was not explicitly provided.
+if (DEFINED USE_HWY AND NOT DEFINED OIIO_USE_HWY)
+    set (OIIO_USE_HWY ${USE_HWY} CACHE BOOL
+         "Enable Google Highway SIMD optimizations (if Highway is available)" FORCE)
+else ()
+    option (OIIO_USE_HWY "Enable Google Highway SIMD optimizations (if Highway is available)" ON)
+endif ()
 
 set (OIIO_LIBNAME_SUFFIX "" CACHE STRING
      "Optional name appended to ${PROJECT_NAME} libraries that are built")