Allow building jpegli against system libjpeg-turbo.

This is an old version of https://github.com/libjxl/libjxl/pull/3085

--- CMakeLists.txt.orig	2024-01-10 09:51:57 UTC
+++ CMakeLists.txt
@@ -81,13 +81,6 @@ endif()
   set(BUNDLE_LIBPNG_DEFAULT NO)
 endif()
 
-
-if(EXISTS "${PROJECT_SOURCE_DIR}/third_party/libjpeg-turbo/jconfig.h.in")
-  set(ENABLE_JPEGLI_DEFAULT YES)
-else()
-  set(ENABLE_JPEGLI_DEFAULT NO)
-endif()
-
 # Standard cmake naming for building shared libraries.
 get_property(SHARED_LIBS_SUPPORTED GLOBAL PROPERTY TARGET_SUPPORTS_SHARED_LIBS)
 option(BUILD_SHARED_LIBS "Build shared libraries instead of static ones" ${SHARED_LIBS_SUPPORTED})
@@ -98,7 +91,7 @@ set(JPEGXL_ENABLE_TOOLS true CACHE BOOL
     "Build JPEGXL developer tools.")
 set(JPEGXL_ENABLE_TOOLS true CACHE BOOL
     "Build JPEGXL user tools: cjxl and djxl.")
-set(JPEGXL_ENABLE_JPEGLI ${ENABLE_JPEGLI_DEFAULT} CACHE BOOL
+set(JPEGXL_ENABLE_JPEGLI true CACHE BOOL
     "Build jpegli library.")
 set(JPEGXL_ENABLE_JPEGLI_LIBJPEG true CACHE BOOL
     "Build libjpeg.so shared library based on jpegli.")
--- lib/jpegli.cmake.orig	2024-01-10 09:51:57 UTC
+++ lib/jpegli.cmake
@@ -23,12 +23,14 @@ endif()
   set(JPEG_LIB_VERSION 80)
 endif()
 
+if(JPEGXL_INSTALL_JPEGLI_LIBJPEG)
 configure_file(
   ../third_party/libjpeg-turbo/jconfig.h.in include/jpegli/jconfig.h)
 configure_file(
   ../third_party/libjpeg-turbo/jpeglib.h include/jpegli/jpeglib.h COPYONLY)
 configure_file(
   ../third_party/libjpeg-turbo/jmorecfg.h include/jpegli/jmorecfg.h COPYONLY)
+endif()
 
 add_library(jpegli-static STATIC EXCLUDE_FROM_ALL "${JPEGXL_INTERNAL_JPEGLI_SOURCES}")
 target_compile_options(jpegli-static PRIVATE "${JPEGXL_INTERNAL_FLAGS}")