build: Avoid secp256k1.h include from system

Author: dergoegge

src/Makefile.am

@@ -20,9 +20,8 @@ else
 LIBUNIVALUE = $(UNIVALUE_LIBS)
 endif
 
-BITCOIN_INCLUDES=-I$(builddir) $(BDB_CPPFLAGS) $(BOOST_CPPFLAGS) $(LEVELDB_CPPFLAGS)
+BITCOIN_INCLUDES=-I$(builddir) -I$(srcdir)/secp256k1/include $(BDB_CPPFLAGS) $(BOOST_CPPFLAGS) $(LEVELDB_CPPFLAGS)
 
-BITCOIN_INCLUDES += -I$(srcdir)/secp256k1/include
 BITCOIN_INCLUDES += $(UNIVALUE_CFLAGS)
 
 LIBBITCOIN_SERVER=libbitcoin_server.a