Support storing assertion messages into core dump

Crash interception/debugging systems like Apport or ABRT capture core dumps for
later crash analysis. However, if a program exits with an assertion failure,
the core dump is not useful since the assertion message is only printed to
stderr.

glibc recently got a patch which stores the message of assert() into the
__abort_msg global variable.
(http://sourceware.org/git/?p=glibc.git;a=commitdiff;h=48dcd0ba)
That works fine for programs which actually use the standard C assert() macro.

This patch adds the same functionality for glib's assertion tests. If we are
building against a glibc which already has __abort_msg (2.11 and later, or
backported above git commit), use that, otherwise put it into our own field
__glib_assert_msg.

Usage:

  $ cat test.c
  #include <glib.h>

  int main() {
      g_assert(1 < 0);
      return 0;
  }

  $ ./test
  **ERROR:test.c:5:main: assertion failed: (1 < 0)
  Aborted (Core dumped)

  $ gdb --batch --ex 'print (char*) __abort_msg' ./test core
  [...]
  $1 = 0x93bf028 "ERROR:test.c:5:main: assertion failed: (1 < 0)"

https://bugzilla.gnome.org/show_bug.cgi?id=594872