Add FLAVORS to allow building jre and headless variants from the same
port. For backwards compatibility, it is also possible to build the jre
version by setting the BUILD_JRE env var.
Also reorganize the Makefile to make portclippy happy
portlint and portclippy don't agree fully on the order in the makefile,
but I hope this is ok.
Sponsored by: The FreeBSD Foundation