gebs_cmd_run_collect() for collecting command output, Add examples/commands.c

gebs.c

@@ -17,6 +17,11 @@ int main(int argc, char ** argv)
         if (GEBS_CMD("gcc", "-ggdb", "-o", "build/arena", "example/arena.c") != 0)
             return 1;
     }
+    if (gebs_needs_rebuild_many("build/commands", "example/commands.c", "gebs.h")) {
+        if (GEBS_CMD("gcc", "-ggdb", "-o", "build/commands", "example/commands.c") != 0) {
+            return 1;
+        }
+    }
 
     return 0;
 }