host-cpu-c-abi: fix detection of MIPS ABI

[Bruno Haible]

With the mips-linux-gnu-gcc-5 cross compiler on Debian,
host-cpu-c-abi reports the n32 ABI, when it's in fact the 32 (a.k.a. o32) ABI.
This fixes it.


2017-07-18  Bruno Haible  <address@hidden>

        host-cpu-c-abi: Fix detection of MIPS ABI.
        * m4/host-cpu-c-abi.m4 (gl_HOST_CPU_C_ABI): For MIPS, really test the
        ABI, not the CPU instruction set.

diff --git a/m4/host-cpu-c-abi.m4 b/m4/host-cpu-c-abi.m4
index a3a3d43..3017f6c 100644
--- a/m4/host-cpu-c-abi.m4
+++ b/m4/host-cpu-c-abi.m4
@@ -1,4 +1,4 @@
-# host-cpu-c-abi.m4 serial 4
+# host-cpu-c-abi.m4 serial 5
 dnl Copyright (C) 2002-2017 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -146,12 +146,12 @@ changequote([,])dnl
             yes
             #endif],
            [gl_cv_host_cpu_c_abi=mips64],
-           [# Strictly speaking, the MIPS ABI (-32 or -n32) is independent
-            # from the CPU identification (-mips[12] or -mips[34]). But -n32
-            # is commonly used together with -mips3, and it's easier to test
-            # the CPU identification.
+           [# In the n32 ABI, _ABIN32 is defined, _ABIO32 is not defined, and
+            # _MIPS_SIM == _ABIN32.
+            # In the 32 ABI, _ABIO32 is defined, _ABIN32 is not defined, and
+            # _MIPS_SIM == _ABIO32.
             AC_EGREP_CPP([yes],
-              [#if __mips >= 3
+              [#if defined _ABIN32
                yes
                #endif],
               [gl_cv_host_cpu_c_abi=mipsn32],