Fix build with libjpeg-turbo

USE_LIBJPEG_TURBO should not be defined since, from pdfiums view, we just
supply a system libjpeg, independent of which implementation is selected.
That define is only if we want to build directly from pdfiums
third_party folder.

cmake/third_party/CMakeLists.txt

@@ -82,12 +82,11 @@ option(PDF_LIBJPEG_TURBO "Use libjpeg-turbo" OFF)
 if(PDF_LIBJPEG_TURBO)
     find_package(libjpeg-turbo)
     target_link_libraries(pdfium_thirdparty_libjpeg INTERFACE libjpeg-turbo::libjpeg-turbo)
-    target_compile_definitions(pdfium_thirdparty_libjpeg INTERFACE USE_LIBJPEG_TURBO=1)
 else()
     find_package(JPEG)
     target_link_libraries(pdfium_thirdparty_libjpeg INTERFACE JPEG::JPEG)
-    target_compile_definitions(pdfium_thirdparty_libjpeg INTERFACE USE_SYSTEM_LIBJPEG)
 endif()
+target_compile_definitions(pdfium_thirdparty_libjpeg INTERFACE USE_SYSTEM_LIBJPEG)
 
 add_library(pdfium_thirdparty_zlib INTERFACE)
 add_library(pdfium::thirdparty_zlib ALIAS pdfium_thirdparty_zlib)