UseJava.cmake: require explicit request to include jars

Modify add_jar to (partly) use cmake_parse_arguments, and to require
using the named argument list INCLUDE_JARS to specify jar files (or jar
targets) to use as dependencies when creating a jar. This preserves the
ability to have such, while restoring the historic behavior that jar
files listed as sources are ignored. (The code now explicitly ignores
them, however, rather than adding them to an unused local variable, so
that it is more clear that nothing is being done with them.)
Merge topic 'use-java-use-parse-arguments' into next

87b3412 UseJava.cmake: require explicit request to include jars