graphics/opencv: Fix JAVA option and correct Apache Ant usage

JAVA option did not work because the actual build knob was wrong.
Especially, on a system where devel/apache-ant was installed, ant and
java binaries were automatically detected and enabled even if the JAVA
option was disabled. While I am here, fix Apache Ant usage by setteing
JAVACMD variable. Without it, it may pick up java binary from JRE via
javavm(1) from java/javavmwrapper. If ant script is used with java from
JRE, build fails because it cannot find jar.

Approved by: tcberner (maintainer)