lablgtk fails to compile with gcc-10

[atupone]

GCC will reject multiple definition of global variables starting from gcc-10

A fix could be:
--- a/src/ml_gdk.h 2020-05-14 22:19:59.045394874 +0200
+++ b/src/ml_gdk.h 2020-05-14 22:20:06.694277819 +0200
@@ -102,7 +102,7 @@
CAMLexport int OptFlags_GdkModifier_val (value);
CAMLexport int Flags_GdkModifier_val (value);
CAMLexport int Flags_Event_mask_val (value);
-CAMLexport lookup_info *ml_table_extension_events;
+extern lookup_info *ml_table_extension_events;
#define Extension_events_val(key) ml_lookup_to_c(ml_table_extension_events,key)

#define GdkDragContext_val(val) check_cast(GDK_DRAG_CONTEXT,val)
--- a/src/ml_gdk.c 2020-05-14 22:21:39.244861128 +0200
+++ b/src/ml_gdk.c 2020-05-14 22:21:57.571580518 +0200
@@ -55,6 +55,8 @@
#define GDK_WINDOW_TYPE_HINT_DOCK GDK_WINDOW_TYPE_HINT_NORMAL
#endif

+lookup_info *ml_table_extension_events;
+
CAMLprim void ml_raise_gdk (const char *errmsg)
{
static const value * exn = NULL;