re-enable mingw appveyor ci testing

.appveyor.yml

@@ -93,6 +93,20 @@ environment:
       B2_CXXSTD: 03,11,14,1z
       B2_TOOLSET: gcc
 
+    - FLAVOR: mingw32
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      B2_ADDRESS_MODEL: 32
+      ADDPATH: C:\mingw\bin;
+      B2_CXXSTD: 03,11,14,1z
+      B2_TOOLSET: gcc
+
+    - FLAVOR: mingw64
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      ADDPATH: C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\bin;
+      B2_ADDRESS_MODEL: 64
+      B2_CXXSTD: 03,11,14,17,2a
+      B2_TOOLSET: gcc
+
 install:
   - git clone --depth 1 https://github.com/boostorg/boost-ci.git C:\boost-ci-cloned
   # Copy ci folder if not testing Boost.CI