tests: avoid some SunOS declaration conflicts.

tests: avoid some SunOS declaration conflicts.

This change: finishes fixing https://bugs.gnu.org/34151
  and https://bugs.gnu.org/30612 and https://bugs.gnu.org/42393;
fixes https://bugs.gnu.org/44795 and https://bugs.gnu.org/49755;
partially fixes https://bugs.gnu.org/45205
  and https://bugs.gnu.org/55073.
Still open: https://bugs.gnu.org/15256;

* t/lex-clean-cxx.sh: test __cplusplus, __sun, and __EXTERN_C__
before using extern "C" for the yylex decl, as explained at
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=45205#13.
* t/lex-depend-cxx.sh: likewise.

* t/yacc-cxx.sh: "using" std::exit, std::free, std::malloc.
* t/yacc-d-cxx.sh: likewise.
* t/yacc-mix-c-cxx.sh: likewise.

* NEWS: mention this.