configure couldn't enable native coroutine correctly if with parameter --target

I am trying to build ruby 2.6.3 downloaded from github.

Once I executed "configure --target x86_64-clr-linux-gnu", configure couldn't enable the native coroutine for amd64, because the target_os is recognized as "linux-gnu", not "linux".

In configure.ac, for enable native coroutine for amd64, target_os have to be "linux",

       AS_CASE(["$target_cpu-$target_os"],
                       [x*64-darwin*], [
                           rb_cv_coroutine=amd64
                       ],
                       [x*64-linux], [
                           AS_CASE(["$ac_cv_sizeof_voidp"],
                               [8], [ rb_cv_coroutine=amd64 ],
                               [4], [ rb_cv_coroutine=x86 ],
                               [*], [ rb_cv_coroutine= ]
                           )
                       ],

but if with --target xxxx, the sed command in the following section (from configure source code) shouldn't work.
the sed command should replace linux-gnu to linux.

# The aliases save the names the user supplied, while $host etc.
# will get canonicalized.
test -n "$target_alias" &&
  test "$program_prefix$program_suffix$program_transform_name" = \
    NONENONEs,x,x, &&
  program_prefix=${target_alias}-
test x"$target_alias" = x &&
target_os=`echo $target_os | sed 's/linux-gnu$/linux/;s/linux-gnu/linux-/'`

so the question is if the match patten code in configure.ac is not enough for matching all linux OS alias?
or I couldn't use the parameter --target when call configure.

thank you

Jie HE