QA script error (libc++)

[Fernando Apesteguía]

One of my ports is written in C++. It links agains libc++ that is in
base (/usr/src/contrib/libc++). The port still builds fine and works
but the QA scripts show an error complaining about the executable
being linked to libc++ without the library being listed as an actual
dependency and it suggests to add the following line to the Makefile:

LIB_DEPENDS+=libc++.so:devel/libc++

Is this strictly necessary? Would something like this be acceptable?:

.if !exists(/usr/lib/libc++.so)
...
LIB_DEPENDS+=libc++.so:devel/libc++
...
.endif

Note: the port does not compile on FreeBSD < 10.x

Thanks in advance.