patch 8.2.2586: process id may be invalid

Problem:    Process id may be invalid.
Solution:   Use sysinfo.uptime to check for recent reboot. (suggested by Hugo
            van der Sanden, closes #7947)
diff --git a/src/config.h.in b/src/config.h.in
index 5d01e2c..fbf4b24 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -213,6 +213,7 @@
 #undef HAVE_SYSCTL
 #undef HAVE_SYSINFO
 #undef HAVE_SYSINFO_MEM_UNIT
+#undef HAVE_SYSINFO_UPTIME
 #undef HAVE_TGETENT
 #undef HAVE_TOWLOWER
 #undef HAVE_TOWUPPER