CMake: only do jpeg-turbo tests if a libjpeg was found

CMakeLists.txt

@@ -58,6 +58,7 @@ endif(WITH_ZLIB)
 
 if(WITH_JPEG)
   find_package(JPEG)
+  if(JPEG_FOUND)
     # Check whether the version of libjpeg we found was libjpeg-turbo and print a
     # warning if not.
     set(CMAKE_REQUIRED_LIBRARIES ${JPEG_LIBRARIES})
@@ -91,6 +92,7 @@ if(WITH_JPEG)
     if(NOT FOUND_LIBJPEG_TURBO)
       message(WARNING "*** The libjpeg library you are building against is not libjpeg-turbo.  Performance will be reduced.  You can obtain libjpeg-turbo from:  https://sourceforge.net/projects/libjpeg-turbo/files/ ***")
     endif()
+  endif(JPEG_FOUND)
 endif(WITH_JPEG)