$OpenBSD$
Index: GNUmakefile
--- GNUmakefile.orig
+++ GNUmakefile
@@ -7,6 +7,11 @@ uname_O := $(shell sh -c 'uname -o 2>/dev/null || echo
 prefix := /usr/local
 bindir := $(prefix)/bin
 
+ifeq ($(uname_S),OpenBSD)
+CFLAGS += -pthread
+LDFLAGS += -pthread
+endif
+
 ifneq ($(findstring MINGW,$(uname_S)),)
 win32 = Yep
 endif