Specify AM_CPPFLAGS for ZMQ.

When building the ZMQ static library, add AM_CPPFLAGS to the library
CPPFLAGS.  Otherwise, we may miss important flags that are specified
elsewhere.  For instance, if --enable-debug is passed and
-DDEBUG_LOCKORDER set, then that would not apply to the ZMQ library
before (causing potential for hard-to-find bugs).

Author: domob1812

src/Makefile.am

@@ -309,7 +309,7 @@ libbitcoin_server_a_SOURCES += dummywallet.cpp
 endif
 
 if ENABLE_ZMQ
-libbitcoin_zmq_a_CPPFLAGS = $(BITCOIN_INCLUDES) $(ZMQ_CFLAGS)
+libbitcoin_zmq_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(ZMQ_CFLAGS)
 libbitcoin_zmq_a_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
 libbitcoin_zmq_a_SOURCES = \
   zmq/zmqabstractnotifier.cpp \