patch 8.0.0445: getpgid is not supported on all systems

Problem:    Getpgid is not supported on all systems.
Solution:   Add a configure check.
diff --git a/src/os_unix.c b/src/os_unix.c
index 6c5955b..9dd5c34 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -3081,7 +3081,7 @@
 }
 
 /*
- * Return 1 if "name" can be found in $PATH and executed, 0 if not.
+ * Return TRUE if "name" can be found in $PATH and executed, FALSE if not.
  * If "use_path" is FALSE only check if "name" is executable.
  * Return -1 if unknown.
  */
@@ -5429,8 +5429,10 @@
 
     /* TODO: have an option to only kill the process, not the group? */
     job_pid = job->jv_pid;
+#ifdef HAVE_GETPGID
     if (job_pid == getpgid(job_pid))
 	job_pid = -job_pid;
+#endif
 
     kill(job_pid, sig);