Fix _Unwind_GetIPInfo detection on Mac OS X 10.4

[Misty De Meo <mistydemeo at gmail dot com>]

Revision 192853 added a new test for availability of _Unwind_GetIPInfo
in the system unwinder to the configure script of libbacktrace:
http://repo.or.cz/w/official-gcc.git/commitdiff/a4a5a77adfc9c28d6963e5ae054c997d57cfc7fa
It was apparently added to fix a bug building with GCC 4.0 on Mac OS X
10.5.

This is one of a few checks for that function (there's also one in the
top-level configure script). Unfortunately, while the other tests have
special handling for Mac OS X 10.4, this new test does not take it
into account.

In 10.4, _Unwind_GetIPInfo is not exported in the system unwinder.
Trying to build a test program will succeed, but will fail to link due
to the symbol being undefined. The test added to libbacktrace's
configure used AC_COMPILE_IFELSE, which meant it didn't try to link
and erroneously flagged _Unwind_GetIPInfo as usable. This led to build
errors when linking.

This small patch changes configure to use AC_LINK_IFELSE, which fixes
the issue on OS X 10.4. I've been able to build GCC successfully.

GCC 4.8 and 4.9 are affected by this issue.

A bug report is filed for this as #58710
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58710).

diff --git libbacktrace/configure.ac libbacktrace/configure.ac
index 28b2a1c..e0e0e08 100644
--- libbacktrace/configure.ac
+++ libbacktrace/configure.ac
@@ -144,7 +144,7 @@ else
   ac_save_CFFLAGS="$CFLAGS"
   CFLAGS="$CFLAGS -Werror-implicit-function-
declaration"
   AC_MSG_CHECKING([for _Unwind_GetIPInfo])
-  AC_COMPILE_IFELSE(
+  AC_LINK_IFELSE(
     [AC_LANG_PROGRAM(
        [#include "unwind.h"
     struct _Unwind_Context *context;