Allow using Java in a cross-compilation toolchain

Since Java is a portable language, if you want to use Java when
cross-compiling, UseJava uses the JVM installed on the host.  So in this
case, we must use set CMAKE_JAVA_INCLUDE_FLAG_SEP for the current host.
Merge topic 'UseJava-cross-compile' into next

778aacc Allow using Java in a cross-compilation toolchain