build: warn when configure with --with-tls=openssl

When expliciting openssl as tls implementation to
use, show a warning as there are still some pieces
of code that may depend on NSS or GNUTLS.

Change-Id: I2cfaaf09b5ad83837928f2648975db956b261c19
Reviewed-on: https://gerrit.libreoffice.org/3327Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>

configure.ac

@@ -8109,6 +8109,10 @@ if test -n "$with_tls"; then
     openssl)
         AC_DEFINE(USE_TLS_OPENSSL)
         TLS=OPENSSL
+
+        # warn that OpenSSL has been selected but not all TLS code has this option
+        AC_MSG_WARN([TLS/SSL implementation to use is OpenSSL but some code may still be depending on NSS or GNUTLS])
+        add_warning "TLS/SSL implementation to use is OpenSSL but some code may still be depending on NSS or GNUTLS"
         ;;
     nss)
         AC_DEFINE(USE_TLS_NSS)