rename 'stdout' variable to 'output', on OpenBSD stdout is a macro defined in stdio.h

and build fails with:

exo-execute.c:136:10: error: expected identifier or '('
  gchar *stdout = NULL;
         ^
/usr/include/stdio.h:198:17: note: expanded from macro 'stdout'

error: cannot take the address of an rvalue of type 'FILE *' (aka 'struct __sFILE *')
  if (g_spawn_command_line_sync(query, &stdout, NULL, NULL, NULL))
2 jobs for !6 with fix-stdout in 2 minutes and 47 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #1384
build

00:00:59

 
  Distcheck
passed #1385
distcheck

00:01:48