Question about macro _GLIBCXX_RES_LIMITS in libstdc++ testsuite

[Bin.Cheng]

Hi,
In libstdc++ testsuite, I noticed that macro _GLIBCXX_RES_LIMITS is
checked/set by GLIBCXX_CHECK_SETRLIMIT, which is further guarded by
GLIBCXX_IS_NATIVE as below:

AC_DEFUN([GLIBCXX_CONFIGURE_TESTSUITE], [
  if $GLIBCXX_IS_NATIVE ; then
    # Do checks for resource limit functions.
    GLIBCXX_CHECK_SETRLIMIT

    # Look for setenv, so that extended locale tests can be performed.
    GLIBCXX_CHECK_STDLIB_DECL_AND_LINKAGE_3(setenv)
  fi

For cross toolchain like arm-linux, _GLIBCXX_RES_LIMITS isn't set.  As
a result, function __gnu_test::set_file_limit is actually nullified
and causing case 27_io/fpos/14775.cc failed.

My question is why we want to guard the check with GLIBCXX_IS_NATIVE?
Could we check it directly, if it's not supported, it's going to fail
and undef the macro anyway?


Thanks,
bin