Index: lib/mkmf.rb
===================================================================
--- lib/mkmf.rb	(revision 45341)
+++ lib/mkmf.rb	(working copy)
@@ -1895,7 +1895,7 @@
 OUTFLAG = #{OUTFLAG}$(empty)
 COUTFLAG = #{COUTFLAG}$(empty)
 
-RUBY_EXTCONF_H = #{$extconf_h}
+RUBY_EXTCONF_H = $(srcdir)/#{$extconf_h}
 cflags   = #{CONFIG['cflags']}
 optflags = #{CONFIG['optflags']}
 debugflags = #{CONFIG['debugflags']}