3rdparty: libbacktrace: sort entries in config.h to keep it the same between reconfigurations

Multicheck parallel nature caused config.h entries to be sorted randomly and
libbacktrace recompiled on every occasion.

3rdparty/libbacktrace/wscript

@@ -2,6 +2,7 @@
 # encoding: utf-8
 
 from waflib import TaskGen
+from waflib.Tools.c_config import DEFKEYS
 
 FRAGMENT_ATOMIC='''int i;
 int main(void) {
@@ -138,6 +139,8 @@ def configure(conf):
 		msg='Checking for in parallel'
 	)
 
+	conf.env[DEFKEYS].sort()
+
 	conf.write_config_header()
 
 	conf.define('BACKTRACE_SUPPORTED', 1)