musl - gnu autoconf AC_CANONICAL_BUILD returns linux-gnu build

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Date: Mon, 30 Dec 2019 14:24:56 +0300
From: Scherbatiy Alexander <alexander.scherbatiy@...l-sw.com>
To: musl <musl@...ts.openwall.com>
Subject: gnu autoconf AC_CANONICAL_BUILD returns linux-gnu build_os on Alpine Linux 3.10.3

Hello,

gnu autoconf AC_CANONICAL_BUILD returns linux-gnu build_os variable on Alpine Linux 3.10.3.

Here is a snippet from the configure.ac file which I tested in docker alpine:3.10.3
----------------
AC_CANONICAL_BUILD
AC_MSG_NOTICE([build_os=$build_os])
----------------
Output:
----------------
checking build system type... x86_64-pc-linux-gnu
configure: build_os=linux-gnu
----------------

The same code on Alpine 3.8.4 (docker alpine:3.8 ) returns:
----------------
checking build system type... x86_64-pc-linux-musl
configure: build_os=linux-musl
----------------

Is it expected that build_os is detected as linux-gnu instead of linux-musl on Alpine 3.0.3?

Thanks,
Alexander.

Simple project:

--- configure.ac ---
AC_INIT([amhello], [1.0], [bug-automake@....org])
AM_INIT_AUTOMAKE([-Wall -Werror foreign])

# Environment
AC_CANONICAL_BUILD
AC_MSG_NOTICE([build_os=$build_os])

AC_PROG_CC
AC_CONFIG_HEADERS([config.h])
AC_CONFIG_FILES([
 Makefile
])
AC_OUTPUT

--- Makefile.am ---
bin_PROGRAMS = hello
hello_SOURCES = main.c

--- Makefile.am ---
bin_PROGRAMS = hello
hello_SOURCES = main.c
/tmp/sample/autoconf-sample # cat main.c 
#include <config.h>
#include <stdio.h>

int main (void)
{
  printf("Hello World!\n");
  return 0;
}

----------------
autoreconf -i
./configure

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