patch 8.2.4457: the GPM library can only be linked statically

Problem:    The GPM library can only be linked statically.
Solution:   Make it possible to load the GPM library dynamically. (Damien)
diff --git a/src/config.h.in b/src/config.h.in
index 20339de..2653e8d 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -507,3 +507,6 @@
 
 /* Define if _SC_SIGSTKSZ is available via sysconf() */
 #undef HAVE_SYSCONF_SIGSTKSZ
+
+/* Define if you want to load libgpm dynamically */
+#undef DYNAMIC_GPM