host-os: Add support for MidnightBSD

[Bruno Haible]

Currently, on MidnightBSD, the 'host-os' module returns "Midnightbsd"; this is
not right.


2021-02-07  Bruno Haible  <bruno@clisp.org>

        host-os: Add support for MidnightBSD.
        * m4/host-os.m4 (gl_HOST_OS): On MidnightBSD, set HOST_OPERATING_SYSTEM
        to "MidnightBSD", not "Midnightbsd".

diff --git a/m4/host-os.m4 b/m4/host-os.m4
index 543473c..813133b 100644
--- a/m4/host-os.m4
+++ b/m4/host-os.m4
@@ -1,4 +1,4 @@
-# serial 10
+# serial 11
 
 # Copyright (C) 2001, 2003-2004, 2006, 2009-2021 Free Software Foundation, Inc.
 # This file is free software; the Free Software Foundation
@@ -41,6 +41,7 @@ AC_DEFUN([gl_HOST_OS],
        netbsd*-gnu*)    os='GNU/NetBSD';; # NetBSD kernel+libc, GNU userland
        netbsd*)         os='NetBSD';;
        mirbsd*)         os='MirBSD';;
+       midnightbsd*)    os='MidnightBSD';;
        knetbsd*-gnu)    os='GNU/kNetBSD';; # NetBSD kernel, GNU libc+userland
        kfreebsd*-gnu)   os='GNU/kFreeBSD';; # FreeBSD kernel, GNU libc+userland
        msdosdjgpp*)     os='DJGPP';;