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/runtime/doc/various.txt b/runtime/doc/various.txt
index 5b35dca..9ca6517 100644
--- a/runtime/doc/various.txt
+++ b/runtime/doc/various.txt
@@ -410,6 +410,7 @@
 N  *+mouseshape*	|'mouseshape'|
 B  *+mouse_dec*		Unix only: Dec terminal mouse handling |dec-mouse|
 N  *+mouse_gpm*		Unix only: Linux console mouse handling |gpm-mouse|
+m  *+mouse_gpm/dyn*	Same as |+mouse_gpm| with optional library dependency |/dyn|
 N  *+mouse_jsbterm*	JSB mouse handling |jsbterm-mouse|
 B  *+mouse_netterm*	Unix only: netterm mouse handling |netterm-mouse|
 N  *+mouse_pterm*	QNX only: pterm mouse handling |qnx-terminal|