textproc/opensp: fix build with clang 16

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

Recognizer.cxx:42:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
  register const Trie *pos = trie_.pointer();
  ^~~~~~~~~

To work around the error, define the register keyword away using
CPPFLAGS.

PR: 271851
Approved by: fernape
MFH: 2023Q2