[wip/jafar-kernel] Only generate the backtrace on linux

To compute the backtrace, we use the execinfo API, which is available
by default only on Linux + Glibc. So, desactive the associated code on
platform != linux

jafar-kernel/distinfo

 SHA1 (jafar-kernel-0.3.tar.gz) = 0dbf0bffa2fda89fbf8036c6cce8952798096a64
 RMD160 (jafar-kernel-0.3.tar.gz) = 56b06fd70ce4ec8896c9ddb08e58a1ac63da8752
 Size (jafar-kernel-0.3.tar.gz) = 94135 bytes
+SHA1 (patch-aa) = f23f565348cf4566426987927e6f33f2770375e8

jafar-kernel/patches/patch-aa

+execinfo.h is, by default, only available on Linux + GLibc. 
+So build it only if we are on linux
+
+--- src/jafarException.cpp.old	2011-12-21 18:22:22.021107884 +0100
++++ src/jafarException.cpp	2011-12-21 18:22:42.521137944 +0100
+@@ -18,12 +18,12 @@
+ namespace kernel {
+ 
+ 		#include <stdlib.h>
+-#ifndef WIN32
++#ifdef __linux__
+ 		#include <execinfo.h>
+ #endif
+ 		std::string get_system_trace ()
+ 		{
+-#ifndef WIN32
++#ifdef __linux__
+ 			void *array[15];
+ 			size_t size;
+ 			char **strings;