Portability fix.

diff --git a/tools/winebuild/Makefile.in b/tools/winebuild/Makefile.in
index af89daf..6c8ea82 100644
--- a/tools/winebuild/Makefile.in
+++ b/tools/winebuild/Makefile.in
@@ -3,6 +3,7 @@
 TOPOBJDIR = ../..
 SRCDIR    = @srcdir@
 VPATH     = @srcdir@
+EXEEXT    = @EXEEXT@
 
 PROGRAMS = winebuild$(EXEEXT)
 MODULE   = none