Make CMake build a test program to see if libjpeg is really available and not broken.

dlib/CMakeLists.txt

@@ -493,16 +493,8 @@ if (NOT TARGET dlib)
       endif()
 
       if (DLIB_JPEG_SUPPORT)
-         # try to find libjpeg 
-         find_package(JPEG QUIET)
-         # 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).
-         if (JPEG_FOUND AND NOT ("${JPEG_INCLUDE_DIR}" MATCHES "(.*)(Ana|ana|mini)conda(.*)") AND NOT BUILDING_PYTHON_IN_MSVC)
-            set(CMAKE_REQUIRED_LIBRARIES ${JPEG_LIBRARY})
-            CHECK_FUNCTION_EXISTS(jpeg_read_header LIBJPEG_IS_GOOD)
-         endif()
-         if (JPEG_FOUND AND LIBJPEG_IS_GOOD AND NOT APPLE)
+         include(cmake_utils/find_libjpeg.cmake)
+         if (JPEG_FOUND)
             include_directories(${JPEG_INCLUDE_DIR})
             set (dlib_needed_libraries ${dlib_needed_libraries} ${JPEG_LIBRARY})
          else()

dlib/cmake_utils/find_libjpeg.cmake

+#This script just runs CMake's built in JPEG finding tool.  But it also checks that the
+#copy of libjpeg that cmake finds actually builds and links.
+
+cmake_minimum_required(VERSION 2.8.12)
+
+# Don't rerun this script if its already been executed.
+if (DEFINED JPEG_FOUND)
+   return()
+endif()
+
+find_package(JPEG QUIET)
+
+if(JPEG_FOUND) 
+   try_compile(test_for_libjpeg_worked 
+      ${PROJECT_BINARY_DIR}/test_for_libjpeg_build  
+      ${CMAKE_CURRENT_LIST_DIR}/test_for_libjpeg
+      test_if_libjpeg_is_broken)
+
+   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.")
+   endif()
+endif()
+
+

dlib/cmake_utils/test_for_libjpeg/CMakeLists.txt

+
+cmake_minimum_required(VERSION 2.8.12)
+project(test_if_libjpeg_is_broken)
+
+find_package(JPEG)
+
+include_directories(${JPEG_INCLUDE_DIR})
+add_executable(libjpeg_test libjpeg_test.cpp)
+target_link_libraries(libjpeg_test ${JPEG_LIBRARY})
+
+

dlib/cmake_utils/test_for_libjpeg/libjpeg_test.cpp

+// Copyright (C) 2019  Davis E. King (davis@dlib.net), Nils Labugt
+// License: Boost Software License   See LICENSE.txt for the full license.
+
+#include <setjmp.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <jpeglib.h>
+#include <iostream>
+
+struct jpeg_loader_error_mgr 
+{
+    jpeg_error_mgr pub;    
+    jmp_buf setjmp_buffer;  
+};
+
+void jpeg_loader_error_exit (j_common_ptr cinfo)
+{
+    jpeg_loader_error_mgr* myerr = (jpeg_loader_error_mgr*) cinfo->err;
+
+    longjmp(myerr->setjmp_buffer, 1);
+}
+
+// This code doesn't really make a lot of sense.  It's just calling all the libjpeg functions to make
+// sure they can be compiled and linked.  
+int main() 
+{
+    std::cerr << "This program is just for build system testing.  Don't actually run it." << std::endl;
+    abort();
+
+    FILE *fp = fopen("whatever.jpg", "rb" );
+
+    jpeg_decompress_struct cinfo;
+    jpeg_loader_error_mgr jerr;
+
+    cinfo.err = jpeg_std_error(&jerr.pub);
+
+    jerr.pub.error_exit = jpeg_loader_error_exit;
+
+    setjmp(jerr.setjmp_buffer);
+
+    jpeg_create_decompress(&cinfo);
+
+    jpeg_stdio_src(&cinfo, fp);
+
+    jpeg_read_header(&cinfo, TRUE);
+
+    jpeg_start_decompress(&cinfo);
+
+    unsigned long height_ = cinfo.output_height;
+    unsigned long width_ = cinfo.output_width;
+    unsigned long output_components_ = cinfo.output_components;
+
+    unsigned char* rows[123];
+
+    while (cinfo.output_scanline < cinfo.output_height)
+    {
+        jpeg_read_scanlines(&cinfo, &rows[cinfo.output_scanline], 100);
+    }
+
+    jpeg_finish_decompress(&cinfo);
+    jpeg_destroy_decompress(&cinfo);
+
+    fclose( fp );
+}