OpenBSD neither provides nor requires librt.

Index: configure
--- configure.orig
+++ configure
@@ -111,7 +111,7 @@ dirlinksym('build/%s/scripts'%host,'install','../src/s
 so = 'dylib' if darwin else 'so'
 so1 = f'so.{soversion}'
 soname = 'install_name' if darwin else 'soname'
-syslibs = '-lm' if darwin else '-lm -lrt'
+syslibs = '-lm'
 
 # ----- compilers
 
