Pass mingw as toolset to boost build on ancient mingw

.appveyor.yml

@@ -28,6 +28,7 @@ branches:
   only:
     - master
     - develop
+    - /appveyor\/.*/
     - /bugfix\/.*/
     - /feature\/.*/
     - /fix\/.*/
@@ -35,6 +36,12 @@ branches:
 
 skip_commits:
   files:
+    - .drone.star
+    - .github/workflows/*
+    - .travis.yml
+    - azure-pipelines/*
+    - drone/*
+    - travis/*
     - LICENSE
     - meta/*
     - README.md
@@ -119,19 +126,28 @@ environment:
       B2_CXXSTD: 11,14,1z
       B2_TOOLSET: gcc
 
+    # disabled: the ancient mingw toolset is not compatible with Boost.Build any more
+    # see: https://ci.appveyor.com/project/cppalliance/boost-ci/builds/52349836/job/vowqt8swcgvnxfpg?fullLog=true#L139
+    # - FLAVOR: mingw32
+    #   APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+    #   B2_ADDRESS_MODEL: 32
+    #   ADDPATH: C:\mingw\bin;
+    #   B2_CXXSTD: 11,14,1z
+    #   B2_TOOLSET: mingw
+
     - FLAVOR: mingw64 (32-bit)
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       ADDPATH: C:\mingw-w64\i686-8.1.0-posix-dwarf-rt_v6-rev0\mingw32\bin;
       B2_ADDRESS_MODEL: 32
       B2_CXXSTD: 11,14,17,2a
-      B2_TOOLSET: gcc
+      B2_TOOLSET: mingw
 
     - 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: 11,14,17,2a
-      B2_TOOLSET: gcc
+      B2_TOOLSET: mingw
 
     - FLAVOR: CodeCov (VS 2019)
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019