java/aparapi: fix recent LLVM6 fallout caused by the switch to default to C++14. As development of aparapi at the location referenced by the port as slowed down, set C++ standard explicitly in the internal Makefile. Ultimately, an upgrade to the more active development location is planned.
Reviewed by: swills (mentor)
Approved by: swills (mentor)
MFH: 2018Q1
Differential Revision: https://reviews.freebsd.org/D14278