Support cross compilation to non-windows targets

Don't have --host/$HOST imply targeting Windows, but actually inspect
the target triple where necessary.

build-mingw-w64-tools.sh

@@ -62,7 +62,11 @@ fi
 if [ -n "$HOST" ]; then
     CONFIGFLAGS="$CONFIGFLAGS --host=$HOST"
     CROSS_NAME=-$HOST
-    EXEEXT=.exe
+    case $HOST in
+    *-mingw32)
+        EXEEXT=.exe
+        ;;
+    esac
 else
     case $(uname) in
     MINGW*)