The last-ever release of uClibc hasn't got prlimit, so probe.
diff --git a/scripts/genconfig.sh b/scripts/genconfig.sh
index 5b0715f..5e7d1df 100755
--- a/scripts/genconfig.sh
+++ b/scripts/genconfig.sh
@@ -90,6 +90,13 @@
     int main(int argc, char *argv[]) { return fork(); }
 EOF
   echo -e '\tdepends on !TOYBOX_MUSL_NOMMU_IS_BROKEN'
+
+  probesymbol TOYBOX_PRLIMIT << EOF
+    #include <sys/time.h>
+    #include <sys/resource.h>
+
+    int main(int argc, char *argv[]) { prlimit(0, 0, 0, 0); }
+EOF
 }
 
 genconfig()
diff --git a/toys/posix/ulimit.c b/toys/posix/ulimit.c
index b14cd93..b4d8e94 100644
--- a/toys/posix/ulimit.c
+++ b/toys/posix/ulimit.c
@@ -21,6 +21,7 @@
 config ULIMIT
   bool "ulimit"
   default y
+  depends on TOYBOX_PRLIMIT
   help
     usage: ulimit [-P PID] [-SHRacdefilmnpqrstuv] [LIMIT]