free tests: Avoid test failure on glibc < 2.15

[Bruno Haible]

When building m4-1.4.18b on a glibc 2.13 system, there is a test failure:

test-free: malloc.c:3551: munmap_chunk: Assertion `ret == 0' failed.
Aborted

This is an internal assertion that was fixed in glibc ([BZ #13276])
on 2011-10-29, that is, before glibc 2.15. Let's avoid this failure,
since we have no way to do anything about it.


2021-05-09  Bruno Haible  <bruno@clisp.org>

        free tests: Avoid test failure on glibc < 2.15.
        * tests/test-free.c (main): Skip the hairy test on glibc < 2.15.

diff --git a/tests/test-free.c b/tests/test-free.c
index e651f18..53f1085 100644
--- a/tests/test-free.c
+++ b/tests/test-free.c
@@ -108,8 +108,10 @@ main ()
      /proc/sys/vm/max_map_count exists.  This file contains the limit
        - for Linux >= 2.4.19: 65536 (DEFAULT_MAX_MAP_COUNT in 
linux/include/linux/sched.h)
        - for Linux >= 2.6.31: 65530 (DEFAULT_MAX_MAP_COUNT in 
linux/include/linux/mm.h).
+     But do not test it with glibc < 2.15, since that triggers a glibc internal
+     abort: "malloc.c:3551: munmap_chunk: Assertion `ret == 0' failed."
    */
-  #if defined __linux__
+  #if defined __linux__ && !(__GLIBC__ == 2 && __GLIBC_MINOR__ < 15)
   if (open ("/proc/sys/vm/max_map_count", O_RDONLY) >= 0)
     {
       /* Preparations.  */