Fix boost headers included as user instead of system headers

In most of the project, boost headers are included as system headers.
Fix the few inconsistent places where they aren't.

configure.ac

@@ -827,14 +827,14 @@ TEMP_CPPFLAGS="$CPPFLAGS"
 CPPFLAGS="$CPPFLAGS $BOOST_CPPFLAGS"
 AC_MSG_CHECKING([for mismatched boost c++11 scoped enums])
 AC_LINK_IFELSE([AC_LANG_PROGRAM([[
-  #include "boost/config.hpp"
-  #include "boost/version.hpp"
+  #include <boost/config.hpp>
+  #include <boost/version.hpp>
   #if !defined(BOOST_NO_SCOPED_ENUMS) && !defined(BOOST_NO_CXX11_SCOPED_ENUMS) && BOOST_VERSION < 105700
   #define BOOST_NO_SCOPED_ENUMS
   #define BOOST_NO_CXX11_SCOPED_ENUMS
   #define CHECK
   #endif
-  #include "boost/filesystem.hpp"
+  #include <boost/filesystem.hpp>
   ]],[[
   #if defined(CHECK)
     boost::filesystem::copy_file("foo", "bar");

src/miner.h

@@ -11,8 +11,8 @@
 
 #include <stdint.h>
 #include <memory>
-#include "boost/multi_index_container.hpp"
-#include "boost/multi_index/ordered_index.hpp"
+#include <boost/multi_index_container.hpp>
+#include <boost/multi_index/ordered_index.hpp>
 
 class CBlockIndex;
 class CChainParams;

src/txmempool.h

@@ -21,11 +21,10 @@
 #include "sync.h"
 #include "random.h"
 
-#include "boost/multi_index_container.hpp"
-#include "boost/multi_index/ordered_index.hpp"
-#include "boost/multi_index/hashed_index.hpp"
+#include <boost/multi_index_container.hpp>
+#include <boost/multi_index/hashed_index.hpp>
+#include <boost/multi_index/ordered_index.hpp>
 #include <boost/multi_index/sequenced_index.hpp>
-
 #include <boost/signals2/signal.hpp>
 
 class CBlockIndex;