Remove all := from BUILD_DEPENDS, here are never needed.

While there, cleanup, and sort depends.

When build and run dependencies are the same, there are three ways to
avoid duplicating the list while not adding the framework added
BUILD_DEPENDS to the RUN_DEPENDS. In order of preference, they are:

1. use RUN_DEPENDS to set BUILD_DEPENDS:

BUILD_DEPENDS= ${RUN_DEPENDS}
RUN_DEPENDS= foo:bar/baz

2. create another variable and use it:

MY_DEPENDS= foo:bar/baz
BUILD_DEPENDS= ${MY_DEPENDS}
RUN_DEPENDS= ${MY_DEPENDS}

3. use BUILD_DEPENDS to set RUN_DEPENDS and force evaluation:

BUILD_DEPENDS= foo:bar/baz
RUN_DEPENDS:= ${BUILD_DEPENDS}

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