Clean the build dirs before building, if the env var CLEAN is set

build-mingw-w64-libraries.sh

@@ -34,6 +34,7 @@ cd mingw-w64/mingw-w64-libraries
 for lib in winpthreads winstorecompat; do
     cd $lib
     for arch in $ARCHS; do
+        [ -z "$CLEAN" ] || rm -rf build-$arch
         mkdir -p build-$arch
         cd build-$arch
         ../configure --host=$arch-w64-mingw32 --prefix="$PREFIX/$arch-w64-mingw32" --libdir="$PREFIX/$arch-w64-mingw32/lib"