Yep, this is an annoying misfeature of Sun Studio. There is no way to
affect this behaviour. There is a Sun Studio bug filed for this
(-errwarn should affect this behaviour, but it doesn't).

Here's some example m4 that libvirt uses to check this case properly:

AC_DEFUN([gl_COMPILER_FLAGS],
  [AC_MSG_CHECKING(whether compiler accepts $1)
   AC_SUBST(COMPILER_FLAGS)
   ac_save_CFLAGS="$CFLAGS"
   CFLAGS="$CFLAGS $1"
   AC_TRY_LINK([], [], has_option=yes, has_option=no,)
   echo 'int x;' >conftest.c
   $CC $CFLAGS -c conftest.c 2>conftest.err
   ret=$?
   if test $ret != 0 -o -s conftest.err -o $has_option = "no"; then
       AC_MSG_RESULT(no)
   else
       AC_MSG_RESULT(yes)
       COMPILER_FLAGS="$COMPILER_FLAGS $1"
   fi
   CFLAGS="$ac_save_CFLAGS"
   rm -f conftest*
 ])