Fix JAVA_OPTS quotes in windows gant.bat start script

Should fix https://jira.codehaus.org/browse/GANT-129
Tried on one of our server with next JAVA_OPTS combinations (note spaces):
set JAVA_OPTS=-Xms900m -Xmx1400m
set JAVA_OPTS=-Xmx1400m
set JAVA_OPTS=