https://github.com/ivmai/bdwgc/commit/808af929bf55cd2b31e354f1903e182b151e8668.patch
Index: patches/patch-include_private_gc_priv_h
===================================================================
RCS file: patches/patch-include_private_gc_priv_h
diff -N patches/patch-include_private_gc_priv_h
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ patches/patch-include_private_gc_priv_h 31 Jan 2021 05:00:46 -0000
@@ -0,0 +1,17 @@
+$OpenBSD$
+
+Fix GC_jmp_buf multiple definition
+808af929bf55cd2b31e354f1903e182b151e8668
+
+Index: include/private/gc_priv.h
+--- include/private/gc_priv.h.orig
++++ include/private/gc_priv.h
+@@ -2515,7 +2515,7 @@ GC_INNER ptr_t GC_store_debug_info(ptr_t p, word sz, c
+
+ #if defined(NEED_FIND_LIMIT) \
+ || (defined(USE_PROC_FOR_LIBRARIES) && defined(THREADS))
+- JMP_BUF GC_jmp_buf;
++ GC_EXTERN JMP_BUF GC_jmp_buf;
+
+ /* Set up a handler for address faults which will longjmp to */
+ /* GC_jmp_buf; */
Index: patches/patch-os_dep_c
===================================================================
RCS file: patches/patch-os_dep_c
diff -N patches/patch-os_dep_c
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ patches/patch-os_dep_c 31 Jan 2021 05:00:46 -0000
@@ -0,0 +1,17 @@
+$OpenBSD$
+
+Fix GC_jmp_buf multiple definition
+808af929bf55cd2b31e354f1903e182b151e8668
+
+Index: os_dep.c
+--- os_dep.c.orig
++++ os_dep.c
+@@ -897,6 +897,8 @@ GC_INNER word GC_page_size = 0;
+ /* Some tools to implement HEURISTIC2 */
+ # define MIN_PAGE_SIZE 256 /* Smallest conceivable page size, bytes */
+
++ GC_INNER JMP_BUF GC_jmp_buf;
++
+ STATIC void GC_fault_handler(int sig GC_ATTR_UNUSED)
+ {
+ LONGJMP(GC_jmp_buf, 1);
