domoticz: fix build with uClibc

domoticz.cpp currently assumes that on GNU/Linux systems header
<execinfo.h> is available. But that is not true. Since it provided by
C library and uClibc can be built without backtrace support. And in
such cases we get following build error.

  domoticz-3.4834/main/domoticz.cpp:48:22: fatal error: execinfo.h: No such file or directory
   #include <execinfo.h>
                        ^
  compilation terminated.

This commit adds patch for detecting presence of <execinfo.h>
and guards code accordingly.

Fixes:

  http://autobuild.buildroot.net/results/393/393f839e160b51ca12ac36058718ad2f0c1b50a6/



Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>