github: Fix win32 MinGW builds (#266)

* github: Log path of g++.exe

* github: Switch ubuntu images to 20.04

18 is no longer supported on Github Actions

* github: Switch to macos-11

Github actions no longer supports macos-10

* github: Switch to clang-10

Github actions ubuntu 20.04 no longer has clang-9 pre-installed.

* github: Attempt number two to print mingw path

* github: Attempt to provide full path to mingw gcc

* github: Fix invalid build yaml

* github: Fix outdated clang9 config name

.github/workflows/build.yml

@@ -100,23 +100,23 @@ jobs:
       matrix:
         config:
         - {
-            image: ubuntu-18.04,
+            image: ubuntu-20.04,
             arch: x86_64,
             os: ubuntu,
             compiler: gcc9,
             cc: gcc-9,
             cxx: g++-9
           }
         - {
-            image: ubuntu-18.04,
+            image: ubuntu-20.04,
             arch: x86_64,
             os: ubuntu,
-            compiler: clang9,
-            cc: clang-9,
-            cxx: clang++-9
+            compiler: clang10,
+            cc: clang-10,
+            cxx: clang++-10
           }
         - {
-            image: macos-10.15,
+            image: macos-11,
             arch: x86_64,
             os: macos,
             compiler: appleclang,
@@ -128,7 +128,9 @@ jobs:
             arch: x86_64,
             os: windows,
             compiler: mingw-w64-7,
-            generator: MinGW
+            generator: MinGW,
+            cc: 'C:\ProgramData\Chocolatey\bin\gcc.exe',
+            cxx: 'C:\ProgramData\Chocolatey\bin\g++.exe'
           }
         - {
             image: windows-2019,
@@ -155,6 +157,7 @@ jobs:
       run: |
         echo "Installing mingw-w64-7"
         choco install mingw --version=7.3.0 --force
+        echo "where g++.exe:"; where.exe g++.exe
         echo "g++.exe --version:"; g++.exe --version
 
     - name: configure-unix