Changed the cmake code for finding libjpeg and libpng slightly

to avoid a cmake error on some platforms when one of these libraries
can't be found.

dlib/CMakeLists.txt

@@ -219,8 +219,10 @@ if (NOT TARGET dlib)
          find_package(PNG QUIET)
          # 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)
+            set(CMAKE_REQUIRED_LIBRARIES ${PNG_LIBRARY})
+            CHECK_FUNCTION_EXISTS(png_create_read_struct LIBPNG_IS_GOOD)
+         endif()
          if (PNG_FOUND AND LIBPNG_IS_GOOD)
             include_directories(${PNG_INCLUDE_DIR})
             set (dlib_needed_libraries ${dlib_needed_libraries} ${PNG_LIBRARY})
@@ -272,8 +274,10 @@ if (NOT TARGET dlib)
          # Make sure there isn't something wrong with the version of libjpeg 
          # installed on this system.  Also don't use the installed libjpeg
          # if this is an APPLE system because apparently it's broken (as of 2015/01/01).
-         set(CMAKE_REQUIRED_LIBRARIES ${JPEG_LIBRARY})
-         CHECK_FUNCTION_EXISTS(jpeg_read_header LIBJPEG_IS_GOOD)
+         if (JPEG_FOUND)
+            set(CMAKE_REQUIRED_LIBRARIES ${JPEG_LIBRARY})
+            CHECK_FUNCTION_EXISTS(jpeg_read_header LIBJPEG_IS_GOOD)
+         endif()
          if (JPEG_FOUND AND LIBJPEG_IS_GOOD AND NOT APPLE)
             include_directories(${JPEG_INCLUDE_DIR})
             set (dlib_needed_libraries ${dlib_needed_libraries} ${JPEG_LIBRARY})