Add -ldl to the build of mttest.c

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/threads/pthread2.sh

diff --git a/crypto/threads/pthread2.sh b/crypto/threads/pthread2.sh
index 41264c6..ec945c4 100755
--- a/crypto/threads/pthread2.sh
+++ b/crypto/threads/pthread2.sh
@@ -3,5 +3,4 @@
 # build using pthreads where it's already built into the system
 #
 /bin/rm -f mttest
-gcc -DPTHREADS -I../../include -g mttest.c -o mttest -L../.. -lssl -lcrypto -lpthread
-
+gcc -DPTHREADS -I../../include -g mttest.c -o mttest -L../.. -lssl -lcrypto -lpthread -ldl