Use the system libjpeg-turbo instead of downloading and statically building
a pinned copy through ExternalProject at build time.

--- a/src/cmake/FlmImageCodecs.cmake
+++ b/src/cmake/FlmImageCodecs.cmake
@@ -1,68 +1,14 @@
 # JPEG/PNG codecs for ImageReader (src/common/image/image_reader.cpp).
 #
-#   - libjpeg-turbo (IJG + BSD-3-Clause + zlib licenses), built as a static
-#     library. Upstream refuses add_subdirectory(), so it goes through
-#     ExternalProject. Only the libjpeg API is built (WITH_TURBOJPEG=OFF): that is
-#     the API Pillow decodes with, and it keeps out the libspng/zlib copy that the
-#     TurboJPEG API bundles. SIMD is used wherever NASM is on PATH.
+#   - libjpeg (the libjpeg API of the system libjpeg-turbo), via pkg-config.
 #   - lodepng (zlib license), vendored in include/lodepng.
 #
 # Defines the INTERFACE target flm_image_codecs. Link it from anything that
 # compiles image_reader.cpp.
 include_guard(GLOBAL)
-include(ExternalProject)
-
-set(FLM_LIBJPEG_TURBO_VERSION 3.2.0)
-set(_flm_ljt_prefix ${CMAKE_BINARY_DIR}/libjpeg-turbo)
-if(MSVC)
-    set(_flm_ljt_lib ${_flm_ljt_prefix}/lib/jpeg-static.lib)
-else()
-    set(_flm_ljt_lib ${_flm_ljt_prefix}/lib/libjpeg.a)
-endif()
-
-set(_flm_ljt_args
-    -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>
-    -DCMAKE_INSTALL_LIBDIR=lib
-    -DCMAKE_BUILD_TYPE=Release
-    -DCMAKE_POSITION_INDEPENDENT_CODE=ON
-    -DENABLE_SHARED=OFF
-    -DENABLE_STATIC=ON
-    -DWITH_TURBOJPEG=OFF
-    -DWITH_TOOLS=OFF
-    -DWITH_TESTS=OFF
-    # MSVC: use the C runtime DLL, like flm, instead of libjpeg-turbo's static default.
-    -DWITH_CRT_DLL=ON)
-if(CMAKE_C_COMPILER)
-    list(APPEND _flm_ljt_args -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER})
-endif()
-
-# Short working directories: the default <name>-prefix/src/<name>-build layout
-# plus CMake's try-compile and MSBuild's .tlog paths exceeds Windows' 260-char
-# MAX_PATH on CI runners with deep work directories.
-set(_flm_ljt_work ${CMAKE_BINARY_DIR}/ljt)
-ExternalProject_Add(flm_libjpeg_turbo
-    URL https://github.com/libjpeg-turbo/libjpeg-turbo/releases/download/${FLM_LIBJPEG_TURBO_VERSION}/libjpeg-turbo-${FLM_LIBJPEG_TURBO_VERSION}.tar.gz
-    URL_HASH SHA256=6f30092cef9fb839779646608f4ee14ae3cbac989c47fa05e841b0841f09878e
-    DOWNLOAD_EXTRACT_TIMESTAMP TRUE
-    EXCLUDE_FROM_ALL TRUE
-    PREFIX ${_flm_ljt_work}
-    DOWNLOAD_DIR ${_flm_ljt_work}/dl
-    SOURCE_DIR ${_flm_ljt_work}/s
-    BINARY_DIR ${_flm_ljt_work}/b
-    STAMP_DIR ${_flm_ljt_work}/st
-    TMP_DIR ${_flm_ljt_work}/t
-    INSTALL_DIR ${_flm_ljt_prefix}
-    CMAKE_ARGS ${_flm_ljt_args}
-    BUILD_BYPRODUCTS ${_flm_ljt_lib})
-
-# The include directory must exist at configure time for the imported target.
-file(MAKE_DIRECTORY ${_flm_ljt_prefix}/include)
-add_library(flm_libjpeg STATIC IMPORTED GLOBAL)
-set_target_properties(flm_libjpeg PROPERTIES
-    IMPORTED_LOCATION ${_flm_ljt_lib}
-    INTERFACE_INCLUDE_DIRECTORIES ${_flm_ljt_prefix}/include)
-add_dependencies(flm_libjpeg flm_libjpeg_turbo)
+find_package(PkgConfig REQUIRED)
+pkg_check_modules(FLM_LIBJPEG REQUIRED IMPORTED_TARGET libjpeg)
 
 add_library(flm_image_codecs INTERFACE)
 target_sources(flm_image_codecs INTERFACE ${CMAKE_CURRENT_LIST_DIR}/../include/lodepng/lodepng.cpp)
-target_link_libraries(flm_image_codecs INTERFACE flm_libjpeg)
+target_link_libraries(flm_image_codecs INTERFACE PkgConfig::FLM_LIBJPEG)