Tune for Windows CI: C11 is broken, so switch to C99 and requests ISO C compatible printf function.

.github/workflows/ccpp.yml

@@ -28,7 +28,7 @@ jobs:
       run:   choco install mingw --no-progress
     - name: make
       shell: bash
-      run: mingw32-make CC="gcc -std=c11"
+      run: mingw32-make CC="gcc -std=c99 -D__USE_MINGW_ANSI_STDIO=1"
     - name: make check
       shell: bash
-      run: mingw32-make CC="gcc -std=c11" check
+      run: mingw32-make CC="gcc -std=c99 -D__USE_MINGW_ANSI_STDIO=1" check