build: add requirement for openssl to configure_ac

Add requirement for openssl to configure_ac so that configure will fail
when not found.

Tested: Verified OPENSSL_LIBS contained libraries.
Change-Id: I255fd4098e804b6db70e5164fa626e23e2894b3f
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index 968d3db..0970c26 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -17,6 +17,6 @@
 	-module \
 	-avoid-version \
 	-shared \
-	-lssl \
+	$(OPENSSL_LIBS) \
 	$(LIBPAM) \
 	$(LIBCRYPT)