Bugfix: Use BOOST_CPPFLAGS when testing for boost sleep vs sleep_for

Author: luke-jr

src/Makefile.include

@@ -8,7 +8,7 @@ endif
 AM_CPPFLAGS =  $(INCLUDES) \
   -I$(top_builddir)/src/obj \
   $(BDB_CPPFLAGS) \
-  $(BOOST_INCLUDES)
+  $(BOOST_CPPFLAGS) $(BOOST_INCLUDES)
 AM_CPPFLAGS += $(LEVELDB_CPPFLAGS)
 AM_LDFLAGS = $(PTHREAD_CFLAGS)