Avoid dynamic initialization of plugin variable in tutorial1 (#113)

example/tutorial1/my_plugin_sum.cpp

@@ -16,9 +16,7 @@ namespace my_namespace {
 
 class my_plugin_sum final: public my_plugin_api {
 public:
-    my_plugin_sum() {
-        std::cout << "Constructing my_plugin_sum" << std::endl;
-    }
+    my_plugin_sum() = default;
 
     std::string name() const override {
         return "sum";
@@ -35,7 +33,7 @@ public:
 
 // Exporting `my_namespace::plugin` variable with name `plugin`
 extern "C" BOOST_SYMBOL_EXPORT my_plugin_sum plugin;
-my_plugin_sum plugin;
+/*constinit*/ my_plugin_sum plugin;
 
 } // namespace my_namespace
 

test/appveyor.yml

@@ -68,10 +68,11 @@ environment:
     #  ADDPATH: C:\mingw\bin;
     #  TOOLSET: gcc
     #  CXXSTD: 11,14,1z
-    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-      ADDPATH: C:\mingw-w64\x86_64-7.2.0-posix-seh-rt_v5-rev1\mingw64\bin;
-      TOOLSET: gcc
-      CXXSTD: 11,14,1z
+    # Not supported by Boost.Atomic any more: `cannot find -lsynchronization`
+    #- APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+    #  ADDPATH: C:\mingw-w64\x86_64-7.2.0-posix-seh-rt_v5-rev1\mingw64\bin;
+    #  TOOLSET: gcc
+    #  CXXSTD: 11,14,1z
 
 before_build:
     - set BOOST_BRANCH=develop