Make sure to use the correct location for config.h when generating event-config.h

extra/libevent/Makefile.am

@@ -22,7 +22,7 @@ include_HEADERS = event.h evutil.h event-config.h
 
 BUILT_SOURCES = event-config.h
 
-event-config.h: $(top_srcdir)/include/config.h
+event-config.h: $(top_builddir)/include/config.h
 	echo '/* event-config.h' > $@
 	echo ' * Generated by autoconf; post-processed by libevent.' >> $@
 	echo ' * Do not edit this file.' >> $@
@@ -33,7 +33,7 @@ event-config.h: $(top_srcdir)/include/config.h
 
 	sed -e 's/#define /#define _EVENT_/' \
 	    -e 's/#undef /#undef _EVENT_/' \
-	    -e 's/#ifndef /#ifndef _EVENT_/' < $(top_srcdir)/include/config.h >> $@
+	    -e 's/#ifndef /#ifndef _EVENT_/' < $(top_builddir)/include/config.h >> $@
 	echo "#endif" >> $@
 
 AM_CPPFLAGS = -Icompat -I$(top_srcdir)/include