Allow building jpegli against system libjpeg-turbo.

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

--- CMakeLists.txt.orig	2024-09-13 05:31:05 UTC
+++ CMakeLists.txt
@@ -83,14 +83,6 @@ else()
   set(BUNDLE_LIBPNG_DEFAULT YES)
 else()
   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)
-  message(STATUS "libjpeg-turbo submodule is absent; not enabling jpegli")
 endif()
 
 include(TestBigEndian)
@@ -112,7 +104,7 @@ 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}")