configure: fix broken test

Since commit d1807a4f836c27f6dc7061e53a834dd27f78e46a ./configure tries
to test files and directories with "test -f", which only test for regular
files. Test with "test -e", which looks for any kind of files.

This unbreak the configure script when not using a separate object
directory.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/configure b/configure
index c058934..54afdda 100755
--- a/configure
+++ b/configure
@@ -3241,7 +3241,7 @@
 done
 mkdir -p $DIRS
 for f in $FILES ; do
-    test -f $f || symlink $source_path/$f $f
+    test -e $f || symlink $source_path/$f $f
 done
 
 # temporary config to build submodules