highway is untagged and have no other consumers. Keep it private to
jpeg-xl package for now.

--- third_party/CMakeLists.txt.orig	2020-11-14 00:52:03 UTC
+++ third_party/CMakeLists.txt
@@ -83,7 +83,7 @@ endif()  # BUILD_TESTING
 # Highway
 if (EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/highway/CMakeLists.txt" AND
     NOT JPEGXL_FORCE_SYSTEM_HWY)
-  add_subdirectory(highway)
+  add_subdirectory(highway EXCLUDE_FROM_ALL)
   configure_file("${CMAKE_CURRENT_SOURCE_DIR}/highway/LICENSE"
                  ${PROJECT_BINARY_DIR}/LICENSE.highway COPYONLY)
 else()
--- third_party/highway/CMakeLists.txt.orig	2020-12-02 19:39:56 UTC
+++ third_party/highway/CMakeLists.txt
@@ -230,7 +230,7 @@ set_target_properties(hwy_benchmark
 
 include(CTest)
 
-if(BUILD_TESTING)
+if(HWY_BUILD_TESTING)
 enable_testing()
 include(GoogleTest)