Fix OpenBSD build with Makefiles

OpenBSD dynamic libraries never link to libc directly.
Instead, they have undefined symbols for all libc functions they use
that ld.so resolves to the libc referred to in the main executable.

Thus, disallowing undefined symbols will always fail

(cherry picked from commit c49bff2434)

mk/libraries.mk

@@ -86,7 +86,9 @@ define build-library
     else
       ifndef HOST_DARWIN
         ifndef HOST_WINDOWS
-          $(1)_LDFLAGS += -Wl,-z,defs
+	  ifndef HOST_OPENBSD
+            $(1)_LDFLAGS += -Wl,-z,defs
+          endif
         endif
       endif
     endif

mk/platform.mk

@@ -21,6 +21,10 @@ ifdef HOST_OS
     HOST_NETBSD = 1
     HOST_UNIX = 1
   endif
+  ifeq ($(patsubst openbsd%,,$(HOST_KERNEL)),)
+    HOST_OPENBSD = 1
+    HOST_UNIX = 1
+  endif
   ifeq ($(HOST_KERNEL), linux)
     HOST_LINUX = 1
     HOST_UNIX = 1