build: support -Dclean

h2/build.bat

 @echo off
 if "%JAVA_HOME%"=="" echo Error: JAVA_HOME is not defined.
+if "%1"=="-Dclean" rmdir /s /q temp | rmdir /s /q bin
 if not exist temp mkdir temp
 if not exist bin mkdir bin
 if exist bin/org/h2/build/Build.class goto buildOK

h2/build.sh

@@ -6,6 +6,9 @@ if [ -z "$JAVA_HOME" ] ; then
     echo "Error: JAVA_HOME is not defined."
   fi
 fi
+if [ $1 == "-Dclean" ] ; then
+  rm -rf temp bin
+fi
 if [ ! -d "temp" ] ; then
   mkdir temp
 fi