WITH_CCACHE_BUILD: Don't leak ccache in as a runtime-dependency.

fmake and bmake's := feature does not evaluate a variable immediately
if it does not yet exist (bmake's manpage notes this). In this case
BUILD_DEPENDS is empty but later gets ccache added to it. So when
RUN_DEPENDS is finally evaluated it actually gets ccache in it.

This line was not useful anyhow since there were not BUILD_DEPENDS to add
in; LIB_DEPENDS is its own unique thing that works as a BUILD and RUN
dependency but not directly related to the other variables.

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