autogen.sh: Fix building when freetype is itself a git submodule.

There are situations where .git is not a directory, like when
freetype is a submodule in another repository.
diff --git a/autogen.sh b/autogen.sh
index 1545c63..b5fc1be 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -182,7 +182,7 @@
   cp $DLG_SRC_DIR/* src/dlg
 }
 
-if test -d ".git"; then
+if test -e ".git"; then
   DLG_INC_DIR=subprojects/dlg/include/dlg
   DLG_SRC_DIR=subprojects/dlg/src/dlg