Improve check that the system version of libjpeg is usable.

dlib/cmake_utils/find_libjpeg.cmake

@@ -31,7 +31,7 @@ if(JPEG_FOUND)
    message (STATUS "Found system copy of libjpeg: ${JPEG_LIBRARY}")
    if(NOT test_for_libjpeg_worked)
       set(JPEG_FOUND 0)
-      message (STATUS "System copy of libjpeg is broken.  Will build our own libjpeg and use that instead.")
+      message (STATUS "System copy of libjpeg is broken or too old.  Will build our own libjpeg and use that instead.")
    endif()
 endif()
 

dlib/cmake_utils/test_for_libjpeg/libjpeg_test.cpp

@@ -41,6 +41,10 @@ int main()
     jpeg_create_decompress(&cinfo);
 
     jpeg_stdio_src(&cinfo, fp);
+    if (false) {
+        unsigned char imgbuffer[1234];
+        jpeg_mem_src(&cinfo, imgbuffer, sizeof(imgbuffer));
+    }
 
     jpeg_read_header(&cinfo, TRUE);