OpenBSD neither provides nor requires librt.

Index: configure
--- configure.orig
+++ configure
@@ -103,7 +103,7 @@
 so = 'dylib' if darwin else 'so'
 so1 = soversion
 soname = 'install_name' if darwin else 'soname'
-syslibs = '-lm' if darwin else '-lm -lrt'
+syslibs = '-lm'
 
 # ----- compilers
 
