Add guarded ssl_msg.c to Makefiles

This commit adds the newly created copy ssl_msg.c of ssl_tls.c
to the build system but guards its content by an `#if 0 ... #endif`
preprocessor guard in order to avoid compilation failures resulting
from code duplication. This guard will be removed once the contents
of ssl_tls.c and ssl_msg.c have been made disjoint.
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index d990d43..1544325 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -87,6 +87,7 @@
     ssl_ciphersuites.c
     ssl_cli.c
     ssl_cookie.c
+    ssl_msg.c
     ssl_srv.c
     ssl_ticket.c
     ssl_tls.c