test-simple-atomic.c compilation error on Solaris

["Tom G. Christensen" <tgc@jupiterrise.com>]

Hello,

The test-simple-atomic test is failing to build on Solaris (tested 10 
and older).

Here's the error from Solaris 10:

make[3]: Entering directory 
`/export/home/tgc/tmp/daily_build/gnulib/000-gnulib-simple-363497c93/gltests'
depbase=`echo test-simple-atomic.o | sed 's|[^/]*$|.deps/&|;s|\.o$||'`;\
/usr/tgcware/gcc9/bin/gcc -DHAVE_CONFIG_H -DEXEEXT=\"\" -I. -I.. 
-DGNULIB_STRICT_CHECKING=1 -DIN_GNULIB_TESTS=1 -I. -I. -I.. -I./.. 
-I../gllib -I./../gllib -I/usr/tgcware/include -D_REENTRANT  -g -O2 -MT 
test-simple-atomic.o -MD -MP -MF $depbase.Tpo -c -o test-simple-atomic.o 
test-simple-atomic.c &&\
mv -f $depbase.Tpo $depbase.Po
In file included from ../gllib/unistd.h:40,
                  from ../gllib/stdlib.h:100,
                  from ../gllib/glthread/thread.h:74,
                  from test-simple-atomic.c:43:
/usr/include/unistd.h:542:23: error: macro "yield" passed 1 arguments, 
but takes just 0
   542 | extern void yield(void);
       |                       ^
test-simple-atomic.c:38: note: macro "yield" defined here
    38 | # define yield() gl_thread_yield ()
       |
make[3]: *** [test-simple-atomic.o] Error 1
make[3]: Target `check-am' not remade because of errors.


It needs <unistd.h> before defining the macro.
Same issue as 617bc96d54db37c138bafb1872056de4453a248b.

-tgc