Many platforms do not support backtraces. Fix compile on Solaris platfor...

...m.

This change was tested with Solaris 10 on X86 and SPARC.

More information on Unix Backtrace Support
 - http://www.gnu.org/software/libc/manual/html_node/Backtraces.html

It is not supported on the following platforms:
 - https://www.gnu.org/software/gnulib/manual/html_node/execinfo_002eh.html

        Mac OS X 10.3,
        FreeBSD 6.0,
        NetBSD 5.0,
        OpenBSD 3.8,
        Minix 3.1.8,
        AIX 5.1,
        HP-UX 11,
        IRIX 6.5,
        OSF/1 5.1,
        Solaris 10,
        Cygwin,
        mingw,
        MSVC 9,
        Interix 3.5,
        BeOS.