Made cmake check that libpng and libjpeg actually contain the link symbols they

are supposed to since, on some systems, these libraries aren't installed
correctly and will cause linker errors if used.

dlib/CMakeLists.txt

@@ -213,11 +213,16 @@ if (NOT TARGET dlib)
             )
       endif()
 
+      INCLUDE (CheckFunctionExists)
 
       if (DLIB_LINK_WITH_LIBPNG)
          # try to find libpng 
          find_package(PNG QUIET)
-         if (PNG_FOUND)
+         # Make sure there isn't something wrong with the version of LIBPNG
+         # installed on this system.  
+         set(CMAKE_REQUIRED_LIBRARIES ${PNG_LIBRARY})
+         CHECK_FUNCTION_EXISTS(png_create_read_struct LIBPNG_IS_GOOD)
+         if (PNG_FOUND AND LIBPNG_IS_GOOD)
             include_directories(${PNG_INCLUDE_DIR})
             set (dlib_needed_libraries ${dlib_needed_libraries} ${PNG_LIBRARY})
          else()
@@ -265,7 +270,11 @@ if (NOT TARGET dlib)
       if (DLIB_LINK_WITH_LIBJPEG)
          # try to find libjpeg 
          find_package(JPEG QUIET)
-         if (JPEG_FOUND)
+         # Make sure there isn't something wrong with the version of libjpeg 
+         # installed on this system.  
+         set(CMAKE_REQUIRED_LIBRARIES ${JPEG_LIBRARY})
+         CHECK_FUNCTION_EXISTS(jpeg_read_header LIBJPEG_IS_GOOD)
+         if (JPEG_FOUND AND LIBJPEG_IS_GOOD)
             include_directories(${JPEG_INCLUDE_DIR})
             set (dlib_needed_libraries ${dlib_needed_libraries} ${JPEG_LIBRARY})
          else()