Add OBJCXX and OBJCXXFLAGS to CONFIGURE_ENV.

When compiled on Darwin bitcoin-qt needs two extra files that are written
in ObjC++. These aren't needed on FreeBSD but because of this the ObjC++
compiler is used as linker instead of the C++ compiler. Without defining
OBJCXX clang may be used as C++ compiler while g++ is used as ObjC++
compiler. This may cause link problems.

PR: 207110
Submitted by: olexander.v.melnyk@gmail.com
Approved by: robbak@robbak.com (maintainer, via email)