textproc/jade: fix build with clang 16

Since clang 16 (and gcc 11) the default C++ standard is now gnu++17.
Because textproc/jade's Makefile does not explicitly set its C++
standard, this leads to an error:

Recognizer.cxx:40:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
  register const Trie *pos = trie_.pointer();
  ^~~~~~~~~

Add USE_CXXSTD=gnu++98 to compile for C++98 with GNU extensions instead.

PR: 271494
Approved by: cy (maintainer)
MFH: 2023Q2