Fix Python build for igl_png.

Former-commit-id: 6279685b746d6d5e74aabee597c82b01442f731f

.appveyor.yml

@@ -15,7 +15,7 @@ build_script:
   - cd c:\projects\libigl\tutorial
   - mkdir build
   - cd build
-  - cmake -D "LIBIGL_USE_STATIC_LIBRARY=ON" -D "LIBIGL_WITH_ANTTWEAKBAR=OFF" -G "Visual Studio 14 2015 Win64" ../
+  - cmake -D "LIBIGL_USE_STATIC_LIBRARY=ON" -D "LIBIGL_WITH_ANTTWEAKBAR=OFF" -D "LIBIGL_WITH_EMBREE=OFF" -G "Visual Studio 14 2015 Win64" ../
 #  - cmake -G "Visual Studio 14 2015 Win64" ../
   - set MSBuildLogger="C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
 #  - set MSBuildOptions=/v:m /p:Configuration=Release /logger:%MSBuildLogger%

shared/cmake/CMakeLists.txt

@@ -431,6 +431,7 @@ if(LIBIGL_WITH_PNG)
   list(APPEND LIBIGL_EXTRA_LIBRARIES ${LIBIGL_PNG_EXTRA_LIBRARIES})
   if(LIBIGL_USE_STATIC_LIBRARY)
     CompileIGL_Module("png" "")
+    target_link_libraries(igl_png PRIVATE igl_stb_image)
     target_include_directories(igl_png PRIVATE ${STB_IMAGE_DIR})
     if(NOT APPLE)
       target_include_directories(igl_png PRIVATE "${NANOGUI_DIR}/ext/glew/include")

shared/cmake/CXXFeatures.cmake

@@ -1,7 +1,11 @@
+################################################################################
+
 if(NOT (${CMAKE_VERSION} VERSION_LESS "3.8.0"))
+	# For CMake 3.8 and above, we can use meta features directly provided by CMake itself
     set(CXX11_FEATURES cxx_std_11)
     set(CXX14_FEATURES cxx_std_14)
     set(CXX17_FEATURES cxx_std_17)
+    return()
 endif()
 
 ################################################################################
@@ -19,6 +23,8 @@ set(CXX17_FEATURES
 
 )
 
+################################################################################
+
 # https://cmake.org/cmake/help/v3.1/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.html
 # cxx_aggregate_default_initializers    Aggregate default initializers, as defined in N3605.
 # cxx_alias_templates                   Template aliases, as defined in N2258.