[SV 57674] Use the system default PATH if $PATH is not set

When using execvp() if $PATH is not present in the environment
it will automatically search the system default PATH string.  Emulate
this by passing the system default PATH to find_in_given_path() if
we don't find PATH in the environment.

* src/job.c (child_execute_job): Use confstr(_CS_PATH) if PATH is not
found.
diff --git a/src/job.c b/src/job.c
index 94835c0..c4e7749 100644
--- a/src/job.c
+++ b/src/job.c
@@ -2381,6 +2381,18 @@
           break;
         }
 
+    /* execvp() will use a default PATH if none is set; emulate that.  */
+    if (p == NULL)
+      {
+        size_t l = confstr (_CS_PATH, NULL, 0);
+        if (l)
+          {
+            char *dp = alloca (l);
+            confstr (_CS_PATH, dp, l);
+            p = dp;
+          }
+      }
+
     cmd = (char *)find_in_given_path (argv[0], p, 0);
   }
 
diff --git a/tests/scripts/misc/general4 b/tests/scripts/misc/general4
index 263505e..0077c89 100644
--- a/tests/scripts/misc/general4
+++ b/tests/scripts/misc/general4
@@ -118,4 +118,11 @@
 
 unlink($sname);
 
+# SV 57674: ensure we use a system default PATH if one is not set
+delete $ENV{PATH};
+run_make_test(q!
+a: ; @echo hi
+!,
+              '', "hi\n");
+
 1;