under the current package system, if you "make update" on basic 
	software (such as perl, libpng or tiff), almost every software installed
	will be rebuilt, due to dependency to perl (either runtime dependency
	or build-time dependency).  we used to have BUILD_DEPENDS and such,
	but they are no longer supported (merged into DEPENDS).
	i think we should recover BUILD_DEPENDS and such.

	i don't want to see mozilla-firefox rebuilt every time i update perl...

itojun