Use _GL_ASYNC_SAFE from gnulib.
* gettext-tools/src/plural-eval.h (sigfpe_code): Mark as volatile.
* gettext-tools/src/plural-eval.c (sigfpe_code): Likewise.
(sigfpe_handler): Mark as _GL_ASYNC_SAFE.
* gnulib-local/lib/term-ostream.oo.c (ASYNC_SAFE): Remove macro.
Use _GL_ASYNC_SAFE instead.
