Have CMake build a little test program to see if the system copy of libpng really works.

dlib/CMakeLists.txt

@@ -430,16 +430,8 @@ if (NOT TARGET dlib)
       endif()
 
       if (DLIB_PNG_SUPPORT)
-         # try to find libpng 
-         find_package(PNG QUIET)
-         # Make sure there isn't something wrong with the version of LIBPNG
-         # installed on this system.  Also never link to anything from anaconda
-         # since it's probably broken.
-         if (PNG_FOUND AND NOT ("${PNG_INCLUDE_DIR}" MATCHES "(.*)(Ana|ana|mini)conda(.*)") AND NOT BUILDING_PYTHON_IN_MSVC)
-            set(CMAKE_REQUIRED_LIBRARIES ${PNG_LIBRARIES})
-            CHECK_FUNCTION_EXISTS(png_create_read_struct LIBPNG_IS_GOOD)
-         endif()
-         if (PNG_FOUND AND LIBPNG_IS_GOOD)
+         include(cmake_utils/find_libpng.cmake)
+         if (PNG_FOUND)
             include_directories(${PNG_INCLUDE_DIR})
             set (dlib_needed_libraries ${dlib_needed_libraries} ${PNG_LIBRARIES})
             set(REQUIRES_LIBS " libpng")

dlib/cmake_utils/find_libpng.cmake

+#This script just runs CMake's built in PNG finding tool.  But it also checks that the
+#copy of libpng 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 PNG_FOUND)
+   return()
+endif()
+
+find_package(PNG QUIET)
+
+if(PNG_FOUND) 
+   try_compile(test_for_libpng_worked 
+      ${PROJECT_BINARY_DIR}/test_for_libpng_build  
+      ${CMAKE_CURRENT_LIST_DIR}/test_for_libpng
+      test_if_libpng_is_broken)
+
+   message (STATUS "Found system copy of libpng: ${PNG_LIBRARIES}")
+   if(NOT test_for_libpng_worked)
+      set(PNG_FOUND 0)
+      message (STATUS "System copy of libpng is broken.  Will build our own libpng and use that instead.")
+   endif()
+endif()
+

dlib/cmake_utils/test_for_libpng/CMakeLists.txt

+
+cmake_minimum_required(VERSION 2.8.12)
+project(test_if_libpng_is_broken)
+
+find_package(PNG)
+
+add_executable(libpng_test libpng_test.cpp)
+target_link_libraries(libpng_test ${PNG_LIBRARIES})
+
+

dlib/cmake_utils/test_for_libpng/libpng_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 <png.h>
+#include <setjmp.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <iostream>
+
+void png_loader_user_error_fn_silent(png_structp  png_struct, png_const_charp ) 
+{
+    longjmp(png_jmpbuf(png_struct),1);
+}
+void png_loader_user_warning_fn_silent(png_structp , png_const_charp ) 
+{
+}
+
+// This code doesn't really make a lot of sense.  It's just calling all the libpng functions to make
+// sure they can be compiled and linked.  
+int main() {
+    png_bytep* row_pointers_;
+    png_structp png_ptr_;
+    png_infop info_ptr_;
+    png_infop end_info_;
+
+    FILE *fp = fopen( "whatever.png", "rb" );
+    png_byte sig[8];
+    fread( sig, 1, 8, fp );
+    png_sig_cmp( sig, 0, 8 );
+    png_ptr_ = png_create_read_struct( PNG_LIBPNG_VER_STRING, NULL, &png_loader_user_error_fn_silent, &png_loader_user_warning_fn_silent );
+
+    png_get_header_ver(NULL);
+    info_ptr_ = png_create_info_struct( png_ptr_ );
+    end_info_ = png_create_info_struct( png_ptr_ );
+    setjmp(png_jmpbuf(png_ptr_));
+    png_set_palette_to_rgb(png_ptr_);
+    png_init_io( png_ptr_, fp );
+    png_set_sig_bytes( png_ptr_, 8 );
+    // flags force one byte per channel output
+    int png_transforms = PNG_TRANSFORM_PACKING;
+    png_read_png( png_ptr_, info_ptr_, png_transforms, NULL );
+    png_get_image_height( png_ptr_, info_ptr_ );
+    png_get_image_width( png_ptr_, info_ptr_ );
+    png_get_bit_depth( png_ptr_, info_ptr_ );
+    png_get_color_type( png_ptr_,  info_ptr_ );
+
+    png_get_rows( png_ptr_, info_ptr_ );
+
+    fclose(fp);
+    png_destroy_read_struct(&png_ptr_, &info_ptr_, &end_info_);
+
+    
+    std::cerr << "This program is just for build system testing.  Don't actually run it." << std::endl;
+    abort();
+}