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/feature.h b/src/feature.h
index 65185a2..9564f8e 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -952,6 +952,12 @@
  */
 #if defined(FEAT_NORMAL) && defined(HAVE_GPM)
 # define FEAT_MOUSE_GPM
+/*
+ * +mouse_gpm/dyn   Load libgpm dynamically.
+ */
+# ifndef DYNAMIC_GPM
+// #  define DYNAMIC_GPM
+# endif
 #endif
 
 #if defined(FEAT_NORMAL) && defined(HAVE_SYSMOUSE)