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 (-lssl -lcrypto).
Change-Id: I7cf3c671749801f089054be3828e9c3dea564157
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index 00947ff..b573cdb 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -17,9 +17,8 @@
 	$(SDBUSPLUS_LIBS) \
 	$(PHOSPHOR_DBUS_INTERFACES_LIBS) \
 	$(PHOSPHOR_LOGGING_LIBS) \
-	-lstdc++fs \
-	-lssl \
-	-lcrypto
+	$(OPENSSL_LIBS) \
+	-lstdc++fs
 
 phosphor_certificate_manager_CXXFLAGS = \
 	$(SYSTEMD_CFLAGS) \