This appears to be not tested with autoconf (cmake works). The assignment
to $LIBS changes nothing, which cannot possibly be the intent.
---
cc-ing to original patch author and sign-off
diff --git a/configure.ac b/configure.ac
index 1b94701..c760787 100644
--- a/configure.ac
+++ b/configure.ac
@@ -40,7 +40,7 @@ dnl libmath (for rtl_fm)
AC_CHECK_LIB(m, atan2, [LIBS="$LIBS -lm"])
dnl librealtime (for rtl_test)
-AC_CHECK_LIB(rt, clock_gettime, [LIBS="$LIBS"])
+AC_CHECK_LIB(rt, clock_gettime, [LIBS="$LIBS -lrt"])
# The following test is taken from WebKit's webkit.m4
saved_CFLAGS="$CFLAGS"
