Bug 793514 - Adding version check for gdb.

It seems that older GDB (under version 7) are not handling very well
some common debug information format, in particular DWARF > 3. Such
version of GDB is usually not a problem since it is quite old (more than
10 years old, it would seem) so you don't see it anymore on any modern
GNU/Linux distribution. On FreeBSD on the other hand, it is still
available (probably for license reasons) and even installed by default!
As a consequence, it makes debugging fail, even though LLDB is also
installed by default.

That is even more of a problem because it would seem that GIMP is killed
(most likely by FreeBSD kernel according to the reporter tests) as a
side-effect of GDB failing, which is seriously bad, in particular since
we also use the debug dialog for non-fatal errors (which could therefore
end up killing GIMP as side effect of a bad GDB!).

So I add some GDB version check. I implement this without any dynamic
memory management, as usual, since this needs to happen also during
crash handling where the state is unstable and prone to memory
allocation failure.
I also add gimp_utils_backtrace_available() public API which can be used
by the Preferences.