enable libtls by default

The API/ABI for the LibreSSL 2.1.x series is now fixed, so we can safely
enable libtls it by default. This is useful for new OpenNTPD and
OpenSMTPD releases as well.

ok deraadt@ beck@ sthen@

scripts/travis

@@ -4,7 +4,7 @@ set -e
 ./autogen.sh
 
 if [ "x$ARCH" = "xnative" ]; then
-	./configure --enable-libtls
+	./configure
 	if [ `uname` = "Darwin" ]; then
 		# OS X runs out of resources if we run 'make -j check'
 		make check
@@ -28,6 +28,6 @@ else
 		export PATH=$PATH:/opt/$ARCH/bin
 	fi
 
-	./configure --host=$CPU-w64-mingw32 --enable-libtls
+	./configure --host=$CPU-w64-mingw32
 	make -j
 fi