Give priority to the build directory headers

Not changing anything right now AFAIK. But if for any reason in the
future we end up with two headers with the same name in the source and
build directories, chances are we want to use the one in the build
directory.

Makefile.am

@@ -232,7 +232,7 @@ MAYBE_CORE =
 endif
 
 AM_CFLAGS = $(LIBEVENT_CFLAGS)
-AM_CPPFLAGS = -I$(srcdir)/compat -I$(srcdir)/include -I./include $(SYS_INCLUDES) $(LIBEVENT_CPPFLAGS)
+AM_CPPFLAGS = -I$(srcdir)/compat -I./include -I$(srcdir)/include $(SYS_INCLUDES) $(LIBEVENT_CPPFLAGS)
 AM_LDFLAGS = $(LIBEVENT_LDFLAGS)
 
 GENERIC_LDFLAGS = -version-info $(VERSION_INFO) $(RELEASE) $(NO_UNDEFINED) $(AM_LDFLAGS)