the use of the ruby18 FLAVOR to explicitly build a version for ruby 1.8.
To handle updates, add a @pkgpath marker with the unflavored PKGPATH
to all ruby18 FLAVOR gem/extconf PLISTs.
Remove format-executable CONFIGURE_STYLE, since only one port uses it.
Instead, allow the port Makefile to set GEM_FLAGS, and have ruby.port.mk
append to that if it already exists.
