Add IS_DEPENDED_TARGET to change the behavior of a port when you want to do something else than "install".  For example,
Add IS_DEPENDED_TARGET to change the behavior of a port when you want
to do something else than "install".  For example,

make IS_DEPENDED_TARGET=fetch fetch

will fetch the required distfiles including those of the dependencies
without actually building and installing dependencies.

Also document ECHO_MSG.

Requested by:	paul
Reviewed by:	paul, jhs and others

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