https://github.com/NetBSD/pkgsrc-wip/blob/87d6597e0a2b/libjxl/patches/patch-plugins_gimp_CMakeLists.txt

* adapt to make gimp plugin a separate package

--- CMakeLists.txt.orig	2021-10-29 18:48:36 UTC
+++ CMakeLists.txt
@@ -2,9 +2,12 @@
 #
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
+cmake_minimum_required(VERSION 3.10)
+project(gimp-jxl LANGUAGES C CXX)
 
 find_package(PkgConfig)
 pkg_check_modules(Gimp IMPORTED_TARGET gimp-2.0>=2.10 gimpui-2.0>=2.10)
+pkg_check_modules(JXL IMPORTED_TARGET libjxl libjxl_threads)
 
 if (NOT Gimp_FOUND)
   message(WARNING "Gimp development libraries not found, the Gimp plugin will not be built")
@@ -19,10 +22,10 @@ add_executable(file-jxl WIN32
   file-jxl-save.cc
   file-jxl-save.h
   file-jxl.cc)
-target_link_libraries(file-jxl jxl jxl_threads PkgConfig::Gimp)
+target_link_libraries(file-jxl PkgConfig::JXL PkgConfig::Gimp)
 
 target_include_directories(file-jxl PUBLIC
-    ${PROJECT_SOURCE_DIR})  # for plugins/gimp absolute paths.
+    ${PROJECT_SOURCE_DIR}/../..)  # for plugins/gimp absolute paths.
 
 pkg_get_variable(GIMP_LIB_DIR gimp-2.0 gimplibdir)
 install(TARGETS file-jxl RUNTIME DESTINATION "${GIMP_LIB_DIR}/plug-ins/file-jxl/")