graphics/caffe: respect CXX

Replace %%CXX%% with the actual value of ${CXX}, not just clang++.

This fixes build on GCC architectures.

PR: 238884
Approved by: mat (mentor), portmgr (blanket: ports compliance), eric@camachat.org (maintainer)
Differential Revision: https://reviews.freebsd.org/D20807