science/openbabel: Fix build with llvm16
This was a bit ambiguous as it uses USE_CXXSTD for GCC while gnu++98 for
clang. However this patch was tested on 12.4-RELEASE i386/amd64,
13.1-RELEASE amd64 and MAIN amd64.
Approved by: portmgr (blanket)
Sponsored by: The FreeBSD Foundation