devel/libc++: specific libcxxrt LIB_DEPENDS conditionally

The libcxxrt was unconditionally specified, but pkg(8) would not
register it on FreeBSD 10+ because the libcxxrt.so requirement of the
specification was satisfied by the base libraries.

As a result, Synth would rebuild the libc++ each time becaues there
were more dependency requirements in the port Makefile than the resulting
package, leading Synth to consider the package obsolete.

Since there are already existence tests for setting flags, the solution
is trivial: move LIB_DEPENDS definition inside the conditional blocks.
The resulting package is the same everywhere, and Synth is happy too.

PR: 207756
Submitted by: Andrew Terekhov
Discussed with: dim@, bapt@ (no conclusion, I fixed it myself)