Make build-scripts portable for GitHub Actions

build-scripts/build-windows

@@ -6,12 +6,15 @@ tool=$2
 
 if [[ $tool == mingw ]]; then
     pacman -Sy --noconfirm make base-devel tar zip unzip
-    if [[ $wordsize == 64 ]]; then
-        pacman -Sy --noconfirm mingw-w64-x86_64-toolchain
-        PATH="/c/tools/msys64/mingw64/bin:$PATH"
+    if [ -d /c/msys64 ]; then
+        PATH="/c/msys64/mingw$wordsize/bin:$PATH"
     else
-        pacman -Sy --noconfirm mingw-w64-i686-toolchain
-        PATH="/c/tools/msys64/mingw32/bin:$PATH"
+        if [[ $wordsize == 64 ]]; then
+            pacman -Sy --noconfirm mingw-w64-x86_64-toolchain
+        else
+            pacman -Sy --noconfirm mingw-w64-i686-toolchain
+        fi
+        PATH="/c/tools/msys64/mingw$wordsize/bin:$PATH"
     fi
     g++ -v
 elif [[ $tool == msvc ]]; then

build-scripts/build-windows.bat

@@ -1,5 +1,6 @@
 @echo on
 @rem Usage: build-windows {32|64} {msvc|mingw}
+setlocal ENABLEDELAYEDEXPANSION
 if %2 == msvc (
     if %1 == 64 (
        call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Auxiliary\Build\vcvars64.bat"
@@ -9,8 +10,11 @@ if %2 == msvc (
     choco install zip
     bash ./build-scripts/build-windows %1 %2
 ) else (
-    @rem The vs2017-win2016 pool has an ancient 64-bit-only mingw.
-    @rem Install msys2 so we can get current gcc toolchains.
-    choco install msys2
-    C:\tools\msys64\usr\bin\env.exe MSYSTEM=MINGW64 /bin/bash -l %CD%/build-scripts/build-windows %1 %2
+    if exist C:\msys64 (
+       set MSYS=C:\msys64
+    ) else (
+       choco install msys2
+       set MSYS=C:\tools\msys64
+    )
+    !MSYS!\usr\bin\env.exe MSYSTEM=MINGW64 /bin/bash -l %CD%/build-scripts/build-windows %1 %2
 )