When building python extensions in Visual Studio, never link to any system

copies of libpng or libjpeg, instead, use the copies what come with dlib.  We
are doing this because too many Visual Studio users have busted copies of these
libraries on their machines.  This will make things just always work.

dlib/CMakeLists.txt

@@ -47,6 +47,18 @@ if(has_parent)
    endif()
 endif()
 
+if (COMMAND pybind11_add_module AND MSVC)
+   # True when building a python extension module using Visual Studio.  We care
+   # about this because a huge number of windows users have broken systems, and
+   # in particular, they have broken or incompatibly installed copies of things
+   # like libjpeg or libpng.  So if we detect we are in this mode we will never
+   # ever link to those libraries.  Instead, we link to the copy included with
+   # dlib.
+   set (BUILDING_PYTHON_IN_MSVC true)
+else()
+   set (BUILDING_PYTHON_IN_MSVC false)
+endif()
+
 if (DLIB_IN_PROJECT_BUILD)
 
    # Check if we are being built as part of a pybind11 module. 
@@ -415,7 +427,7 @@ if (NOT TARGET dlib)
          # 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 ("${PNG_INCLUDE_DIR}" MATCHES "(.*)(Ana|ana|mini)conda(.*)"))
+         if (PNG_FOUND AND NOT ("${PNG_INCLUDE_DIR}" MATCHES "(.*)(Ana|ana|mini)conda(.*)") AND NOT BUILDING_PYTHON_IN_MSVC)
             set(CMAKE_REQUIRED_LIBRARIES ${PNG_LIBRARIES})
             CHECK_FUNCTION_EXISTS(png_create_read_struct LIBPNG_IS_GOOD)
          endif()
@@ -484,7 +496,7 @@ if (NOT TARGET dlib)
          # Make sure there isn't something wrong with the version of libjpeg 
          # installed on this system.  Also don't use the installed libjpeg
          # if this is an APPLE system because apparently it's broken (as of 2015/01/01).
-         if (JPEG_FOUND AND NOT ("${JPEG_INCLUDE_DIR}" MATCHES "(.*)(Ana|ana|mini)conda(.*)"))
+         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()