Problem: empty files are against ISO C++

Solution: if not building with tweetnacl, do not include its sources
in Makefile.am. CMake already includes them optionally.

Makefile.am

@@ -192,8 +192,6 @@ src_libzmq_la_SOURCES = \
 	src/tipc_listener.hpp \
 	src/trie.cpp \
 	src/trie.hpp \
-	src/tweetnacl.c \
-	src/tweetnacl.h \
 	src/udp_address.cpp \
 	src/udp_address.hpp \
 	src/udp_engine.cpp \
@@ -232,6 +230,12 @@ src_libzmq_la_SOURCES = \
         src/socket_poller.hpp \
         src/socket_poller.cpp
 
+if USE_TWEETNACL
+src_libzmq_la_SOURCES += \
+	src/tweetnacl.c \
+	src/tweetnacl.h
+endif
+
 if ON_MINGW
 src_libzmq_la_LDFLAGS = \
 	-no-undefined \