Add $(EXEEXT) to executable name to work on windows

Automake defines this for us. Might as well use it.
Should help both mingw and cygwin tests.

testsuite/Makefile.am

@@ -4,7 +4,7 @@
 ##  This file is part of the "Sane" build infra-structure.  See
 ##  included LICENSE file for license information.
 
-SCANIMAGE = ../frontend/scanimage
+SCANIMAGE = ../frontend/scanimage$(EXEEXT)
 TESTFILE  = $(srcdir)/testfile.pnm
 OUTFILE   = outfile.pnm
 DEVICE    = test

testsuite/Makefile.in

@@ -216,7 +216,7 @@ target_alias = @target_alias@
 top_build_prefix = @top_build_prefix@
 top_builddir = @top_builddir@
 top_srcdir = @top_srcdir@
-SCANIMAGE = ../frontend/scanimage
+SCANIMAGE = ../frontend/scanimage$(EXEEXT)
 TESTFILE = $(srcdir)/testfile.pnm
 OUTFILE = outfile.pnm
 DEVICE = test